"characterized by a raised longitudinal rib," differ from those of Ctenodus, which have no such process or "rib."

The palatal

All the tooth-plates described in the sequel of this communication have such a general resemblance to each other that there can be no doubt of the close relationship of the fishes to which they belong. And, moreover, the bones to which many of them are found attached closely resemble each other. tooth (or that which has been so designated) is seated on a broadish flattened bone, which, with one exception, is never more than twice the length of the tooth, and is usually considerably shorter; and it is always much expanded at the posterior extremity. This is probably a maxillary bone; and the two branches, when united, have much the appearance of an upper jaw. The propriety, therefore, of calling these palatal plates or teeth may be questioned. The homologies, however, of these parts must be left for the present untouched.* The mandibular tooth is always much narrower than the palatal; and the branch of the mandibulum on which it is placed is not so wide as the bone supporting the palatal tooth, neither is it so much expanded at the posterior end; it is, however, strong and rather massive.

There are two species that have the surface of the teeth with smooth ridges not tuberculated or denticulated, as they are in all the other kinds. These two have consequently some resemblance to the palatal plates of Ceratodus, apparently a closely allied form. When further research shall have thrown more light on these obscure species, it will then be time enough to consider the desirableness of dividing this apparently natural group into separate genera.

We shall now conclude this brief communication with concise descriptions of the various dental plates that have come under our observation, retaining the denomination of palatal plates or teeth for the upper pair, though the bones to which they are attached have all the appearance of maxillaries, and so they will be named in the following descriptions.

*This matter is undoubtedly discussed in Professor Pander's "Monograph on the Ctenododipterini;" but we have not seen that important work.

1. CTENODUS CRISTATUS, Agassiz.

Poissons Fossiles, Vol. III., p. 137, Pl. XIX., fig. 16.

Tooth plate-like, rather thin, irregularly elliptical, inclining to ovate, 2 inches long, 1 inches broad; the upper surface somewhat hollowed or concave; the inner margin well arched, the outer much less so; the whole surface is covered with twelve close-set transverse ridges, which are studded from end to end with closely arranged conical tubercles; the ridges increase in size as they approach the outer margin, and, being inclined towards the anterior and posterior margins, assume a tendency to a radial disposition; the grooves between the ridges are angulated; the tubercles are perfect only at the outer margin, where they are covered with a coat of brilliant enamel, and here they are seen to have the outer face flattened, the base of each tubercle or denticle being subtriangular; elsewhere they are much worn and somewhat compressed at the sides in the direction of the ridges, the whole surface of the tubercles and ridges exhibiting a coarse irregular granulation.

The above description is of a palatal tooth which has the entire right ramus of apparently the maxillary bone attached. The tooth itself is considerably more than half the length of the bone, the posterior extremity of which is very wide and truncate.

Four or five other specimens have occurred, but they are considerably worn, the tubercles in some instances having almost disappeared; two are mandibular, two palatal; the former are very much narrower than the latter.

There can be little doubt that this is Agassiz's species, though our specimens are scarcely so broad as that represented in the "Poissons Fossiles," and they have likewise the posterior margin a little more produced. All the specimens are from the band or dark shale overlying the Low-Main Coal Seam at Newsham.

2. CTENODUS TUBERCULATUS, n. sp.

Tooth plate-like, thick, with an irregular ovate outline, 2 inches long, 1 inches broad, the narrow end posterior; the inner margin gibbous or angulated in the centre; the outer margin

a little convex; the surface is slightly convex, and is furnished with twelve or thirteen deep, sharp, parallel, approximate ridges, which are strongly tuberculated towards the outer margin, and divided by narrow, deep, angulated grooves; they are arched posteriorly, and enlarged towards the exterior border, but do not at all assume a radial arrangement; the anterior ridge, which is wider than the others, is reflected and prolonged for some distance beyond the outer margin; the tubercles are conical, with obtuse points; those next the external border are coated with shining enamel, and are well produced.

The mandibular tooth differs from the palatal in being narrower, and is so convex as to resemble the longitudinal section of a cylinder; the two or three anterior ridges, too, are much shorter than the rest, the inner margin sloping rather rapidly away in front.

There are half-a-dozen of this species in our collection, all of which were obtained at Newsham; and in the Newcastle Museum there are two others, which were likewise procured from the same locality, and are from the collection of G. B. Forster, Esq. They are all in excellent condition, agree perfectly well in every respect, and can be at once distinguished from C. cristatus by the deep and sharp ridges and by the form of the tubercles, which in C. tuberculatus are always exactly conical (when they are in a fresh state) at the outer margin. When worn, however, they are flattened at the sides in the direction of the ridges; and then they are wedge-shaped, and they and the whole of the ridges become granulated.

3. CTENODUS CORRUGATUS, n. sp.

Tooth plate-like, thin, subtriangular, three inches long, two inches broad; the surface is slightly convex, and raised into nine stout somewhat irregular rounded ridges or wrinkles, the grooves dividing them being wide and rounded; the ridges die out towards the inner and outer margins, but are enlarged a little as they approach the external border, and are indistinctly and irregularly tuberculated; the inner margin is nearly straight, the

outer slightly convex, the anterior slopes forward from the inner margin, and the posterior is produced and rounded. The whole surface is strongly and irregularly punctured.

We have seen only one specimen of this fine large distinct species; it is a palatal tooth, and is in a very good state. It was obtained from the Collingwood Main Pit, and was presented by George Johnson, Esq., to the Newcastle Museum. It is readily distinguished from its allies by the fewness of the ridges, by their roundness and wide separation, as well as by their greater size and general form.

4. CTENODUS OBLIQUUS, n. sp.

Tooth depressed, lanceolate, 1 inches long, ths of an inch broad; the inner margin regularly and much arched, the outer only slightly curved; the surface with six or seven strong, compressed, sharp-edged ridges, placed transversely in a somewhat radiating manner towards the outer margin, where, being enlarged, they are curved downwards and denticulated; the anterior ridges are very oblique, being much inclined forwards; the denticles or tubercles are much compressed in the direction of the ridges, and are lancet-formed, with sharp points; they are coated with brilliant enamel in finely preserved specimens. The above is the description of the palatal tooth; the mandibular is narrower than the former, and is broadest in front, tapering pretty regularly to the posterior extremity; the anterior ridge is very wide, and much produced beyond the outer extremities of the others, and they are all more curved downwards than those of the palatal tooth.

This is a very distinct form, and cannot be confounded with any of the other species. We have collected between forty and fifty specimens of it, most of which were procured from the LowMain shale at Newsham; the rest are from Cramlington. Many of them have the jawbone attached in a very perfect condition. The palatal tooth is a little more than half the length of the bone, the posterior extremity of which is much expanded and

truncated. The mandibular bone is not much widened posteriorly, and is about half as long again as the tooth.

The specimens vary much in size, the largest being a little over 1 inches long, while the length of the smallest is not more than ths of an inch. The former has seven ridges, the latter six; so that it is evident that age does not make any material change in this respect. This is highly satisfactory, as we can with greater assurance assume that the number of ridges is a good specific character.

The anterior portion of a fish of this species has occurred at Newsham, exhibiting one of the gill-opercles and two of the dental plates with the denticles interlocked; numerous scales are also displayed, but they are in such a mashed-up state that the characters cannot be determined with precision. Enough, however, is seen to show that they agree with those of C. elegans, though the ridges or plaits ornamenting the surface are not so numerous, and are considerably coarser; the rosette, too, in the centre of the exposed portion is not so conspicuous, and seems to be smaller. The opercular plate, which is upwards of an inch across its longer diameter, and a little more than ths of an inch in the other direction, is thick and irregularly ovate, with the margin sinuous; the hinge-line is long, and has a slight projection in the centre; the surface is strongly punctured and granulated.

5. CTENODUS ELEGANS, n. sp.

Tooth depressed, triangular, upwards of ths of an inch long, andths of an inch broad; the inner margin is produced and angulated in the centre, whence it slopes in front and behind towards the outer margin, which is gradually but slightly arched; the surface is provided with eight strongly denticulated ridges, which radiate from the inner marginal angle, where they are very minute, to the outer border; the anterior ridge is a little produced at the outer margin beyond the rest; there are six or seven denticles or tubercles on each ridge; they are much compressed at the sides, are sharp pointed and lancet-like, with the inner limb a little shouldered, where there is occasionally a minute toothlet ;