0^ven — On the Teeth of Cochliodonts. 61 



species was originally founded, and is tlie most common among the 

 many detached ' palates ' of the English mountain limestone lo- 

 calities. It is mutilated in both halves of the jaw ; but I have 

 figured more entire specimens of this tooth, from the right ramus of 

 the lower jaw, (PI. III. Fig. 5), and from the left ramus, (PL III. 

 Fig. 4) ; which positions in the jaw, the specimen (Fig. 1) enables 

 one to determine, in detached teeth. 



In this third or posterior grinder (PI. Ill, Fig. 1 c) the longitudinal 

 exceeds the transverse diameter, but the triangular form of the crown 

 prevails, the base now forming the longest side (Fig. 5,^'^'^'), and 

 the apex being more truncate than in the middle tooth. The apex, 

 or outer side of the third tooth, is shown at Fig. 2 c, or rather 

 the vasodentine supporting the apical part of the vitrodentine, 

 which is here very thin, and a small part of it is broken off in 

 the specimen. The mid-lobe, c, as it rises, bends inward and 

 slightly backward, maintaining its convexity as it expands, better 

 than in the tooth h, and terminating, medially or internally, in a 

 gently convex thin border. In the detached homologues of this 

 tooth, from both sides of the jaw, the mid-lobes hows at its 

 inner half a surface worn more or less flat by trituration and 

 sloping from before, outward and backward. The reticulate surface 

 of the osteodentine often takes the place of the punctate surface of the 

 vitrodentine at the middle and toward the back part of this worn 

 surface, showing where the pressure and attrition was greatest from 

 the opposite upper crusher. The anterior lobe of the third tooth, at 

 its apex or outer end, Fig. 2, is almost as broad from behind forward 

 as the middle lobe, but it gains in that diameter very slightly as it 

 curves over the tooth to the inner side, and it is very little elevated, 

 especially in the upper and inner surfaces ; there is, however, some 

 variety in this respect, as in the detached tooth, Fig. 4, but not to a 

 degree which I am disposed to regard as specific. The posterior lobe 

 (Figs. 4, 5*), of equal breadth with the middle one on the outer border 

 (Fig. 2 c), increases in antero-posterior extent, as it curves inward, 

 in a greater degree than does the anterior lobe, but in a less degree 

 than does the middle one ; it soon loses the slight degree of fore- 

 and-aft convexity, with which it began externally, and, from being 

 flat, becomes slightly concave on its inner and broader end. In 

 passing from without inward, the third lobe inclines more backward 

 than either of the preceding lobes, and the anterior lobe the least so, 

 the almost (antero-posteriorly) flattened surface on the third lobe 

 slightly rises toward its posterior border, which is smoothly rounded, 

 Fig. 4». 



The substance of the mandible supporting the teeth equals in 

 vertical extent that of the tooth it supports ; it goes on increasing in 

 depth posteriorly for the extent to which it is preserved, but di- 

 minishes in breadth, indicating a shape of jaw like that in Cestracion. 

 The structure of the bone resembles that of the better ossified 

 parts of the chondrine of plagiostomous fishes. 



The third tooth, right side, lower jaw, of Cochliodus (PL IV, Fig. 

 1) presents specific modifications of form, as compared with that of 



