HYPSODON ETC. FROM BRITISH CRETACEOUS STRATA. 521 



its lower margin, there is a deep depression, as in the genus Por- 

 theus ; and below this the jaw becomes much thinner. The upper 

 and lower margins are nearly parallel throughout the extent of the 

 dentary element ; but behind this the depth of the jaw becomes 

 much reduced, the facet of the articular bone being at a much 

 lower level than the alveolar margin. The symphysis must have 

 resembled that of the Gault Portheus ; and its length could not have 

 been much less than the greatest depth of the jaw. 



The specimen represented by fig. 14 is the left ramus of the jaw 

 figured by Dixon, some of the teeth being completed from the right 

 side. 



ICHTHYODECTES ELEGANS, 11. Sp. Plate XXII. fig. 15. 



There is in the British Museum (No. 41687) a small specimen 

 of a left lower jaw from the Toulmin-Smith collection, which was 

 obtained from the Lower Chalk of Dorking. This specimen mea- 

 sures 3 inches in length in its present condition ; but the articular 

 portion is wanting. When the piece of chalk containing the speci- 

 men was first broken open the jaw itself was split from end to end 

 in such a manner that the roots of all the teeth were exposed. One 

 half was then imbedded in plaster of Paris, and the chalk removed 

 so as to expose the inner surface of the jaw, which is represented 

 in figure 15. Only a few of the teeth are seen in this half; but 

 they have been restored in the figure by reference to the counter- 

 part, in which all the roots and several of the crowns are still pre- 

 served. The upper margin, which appears to be entire, forms a 

 regular curve from the hinder end to near the front, where it is in- 

 terrupted by a projection, similar to that in /. hamatus, Cope. 

 This projection bears a small tooth directed obliquely backwards, 

 and therefore in a different direction from the rest of the teeth, which 

 are inclined forward at a considerable angle. The remains of from 

 twenty -five to twenty-eight teeth can be traced along the jaw ; they 

 are all very nearly of the same size, as shown by the regularity of 

 the fangs, and were hollow, long, and slender, with a decided inward 

 curvature. The symphysis is deep, as in other species of the genus. 

 The lower margin is incomplete. The articular bone is altogether 

 wanting ; but the dentary portion appears to be nearly complete 

 posteriorly. 



This mandible differs from that of any species of Ichthyodectes 

 hitherto described in the curve of its alveolar margin and in the 

 obliquity of its teeth. The mandibles of the American I. progna- 

 thus. Cope, and i". multidentatus, Cope, however, are not at present 

 known ; it is possible therefore that this specimen may belong to 

 one or the other of these species ; but, until this point can be 

 settled by the acquisition of better specimens, it has been deemed 

 advisable to let the English species be known by a separate 

 specific name. 



2m2 



