90 FOSSIL REPTILIA OF THE 



presents its natural form, being traversed longitudinally by a moderate median ridge, 

 on each side of which it is slightly concave transversely. It is perforated by a few 

 small irregular vascular foramina ; but the bony roof of the mouth is continued for an 

 extent of six inches without any trace of its interruption by the naso-palatal aperture. 

 There are no orifices on the inner side of the alveoli : the successional teeth, as 

 will be presently shown, emerge as in the Crocodile, from the old sockets, and not 

 as in certain Mammals and Fishes, by foramina distinct from them. The second 

 and third alveoli are the largest ; the fourth, fifth, and sixth the smallest, yet they 

 are more than half the size of the foregoing ; with which the rest are nearly equal. 

 The outlets of the alveoli are elliptical, and they form prominences at the side of 

 the jaw, or rather the jaw there sinks gently in between the alveoli, and is con- 

 tinued into the bony palate, without any ridge, the vertical wall bending round to 

 form the horizontal plate. The greatest breadth of the under surface of the jaw, 

 taken from the outside of the alveoli, varies only from seven lines across the third 

 pair to nine lines across the eleventh pair of alveoli ; and from this narrow base the 

 sides of the jaw converge with a slight convexity outwards at the anterior half of 

 the fragment, but are almost plane at the deeper posterior half, where they seem to 

 have met at an acute superior ridge ; indeed, such a ridge is continued to within 

 an inch of the fore part of the jaw, where the upper border becomes more obtuse. 



The whole portion of the jaw consists of one uninterrupted bone — the pre- 

 raaxillary ; the delicate crust of osseous substance, as thin as paper, is traversed 

 by many irregular cracks and fissures, but there is no recognizable suture marking off 

 the limits of a maxillary or nasal bone. The bone offers to the naked eye a fine fibrous 

 structure, so fine as to produce almost a silken aspect : the fibres or striae being longi- 

 tudinal, and impressed at intervals of from two to six lines by small vascular foramina. 



The first socket on the right side contains a young tooth which protrudes about a 

 third of an inch obliquely downwards and forwards, (fig. 1 , a .) the fifth socket on the 

 right side and the eighth on the left contain the germ of a younger tooth, the point of 

 which does not protrude beyond the socket ; it lies close to the inner wall of the socket 

 of the old tooth, from which it could have emerged, as in the Crocodile. Two fully 

 developed teeth, (figs. 5 and 6,) are preserved in the same block of chalk with the jaw. 

 One of these is 1 inch 4 lines in length, sabre-shaped, subcompressed, slightly bent, 

 and gradually diminishing in breadth from the widely-open base to the apex : this part 

 is broken off in both specimens, showing the crown to be composed of a compact hard 

 dentine, sheathed by a thin coat of shining enamel : about 9 lines of the basal part of 

 the present tooth, (fig. 5,) is coated by a thin layer of cement. The enamel is marked 

 by extremely fine longitudinal ridges, with an irregular or thready course, of unequal 

 length and with wide intervals, as shown in the magnified view, (fig. 7.) The second, 

 (fig. 6,) is a somewhat smaller tooth ; having the same structure. 



The unique specimen above described was obtained from the Durham Chalk-pit, 

 Kent, and forms part of the fine Collection of James S. Bowerbank, Esq., F.R.S. 



