60 FOSSIL FISHES OF THE ENGLISH CHALK. 



nearly one-quarter of the oral border of the palato-pterygoid arcade, and its hinder 

 half overlaps the ectopterygoid. Its large inner facette for articulation with the 

 cranium extends far forwards, and at the anterior end of this articulation the 

 depth of the bone somewhat exceeds one-third of its total length. Its outer face 

 (fig. 4) is marked with an oblique, rounded depression just below the upper edge 

 of the articulation. The rounded anterior portion of the bone bearing the tooth is 

 very short, but as the extreme end is formed by the bony base of the tooth itself, 

 the exact shape doubtless varies according to the presence of the anterior or the 

 posterior tooth of the two which are alternately in use. The palatine tooth itself 

 is straight and lanceolate, and its length about equals that of the bone which bears 

 it. As shoAvn in transverse section (fig. 6), both its faces are convex, the antero- 

 external face regularly so, the postero-internal face unsymmetrically bulging 

 inwards. The outer view of the tooth (fig. 4) thus shows the one face bounded by 

 the sharp edges ; while the inner view (fig. 5) shows parts of both convex faces 

 divided by the inner sharp edge. Both these faces are marked with delicate longi- 

 tudinal striae, which are especially conspicuous in the original of fig. 5. The 

 maxilla (fig. 1 , mx.) is a very slender bar of bone, not expanded at either end, but 

 marked in its hinder portion by two or three finely-tuberculated longitudinal ridges. 

 Its anterior half is excluded from the gape by the premaxilla, while its posterior 

 half distinctly enters the oral border and seems to have borne minute teeth (B. M. 

 no. P. 5415). The premaxilla is obscured in the original of fig. 1 by matrix and 

 the displaced antorbital plate; but it is shown from the inner aspect by Agassiz, 

 loc. ciL, fig. 1 (right lower corner of specimen), while its outer face is exposed in 

 B. M. no. P. 5415, which is figured in Proc. Geol. Assoc, vol. x, pi. i, fig. 5. It is 

 an irregularly triangular lamina of bone, deepest in front and tapering to a point 

 behind. It seems to have been about twice as long as deep, and its oral margin 

 exhibits a slightly sinuous curve, turning upwards in front. It bears a single series 

 of small conical teeth, which are well spaced and nearly uniform in size, with a 

 tendency towards backward inclination. Its outer face is marked witli a few 

 ridges radiating from a point situated antero-inferiorly, and there are sometimes 

 braces of a tubercular ornament. Where the premaxilla curves inwards in front to 

 meel its fellow of the opposite side, the outer face of the bone is impressed with a 

 deep, vertically extended pit. This is well shown on a fragment in B. M. no. 1*. 

 6459a. The mandible is satisfactorily seen from the outer aspect in fig. 1, and its 

 maximum depth in the coronoid region is shown to equal about one third of its 

 total length. The art icnlo-angular hone (ag.) is relatively large, extending along 

 half the length of the jaw. [ts outer face is ornamented with finely tubercul at ed 

 ridges, which radiate forwards and upwards from the mandibular articulation and 

 are fewest <»ii thecoronoid portion. The deiitarv (d.) tapers rapidly forwards to an 

 almost pointed symphysis, imperfed in fig. 1, hut well seen in a fragmentary 

 detached specimen which is represented iii lig. 7. Below the attenuated symphysis 



