224 BRITISH FOSSIL REPTILES. 



■without the natural anterior termination, and wanting all that extensive hinder part 

 of the ramus formed by the angular and surangular pieces. The inner alveolar plate 

 of the dentary is broken away ; but the vertical diameter of the outer part of the bone, 

 from being 2 inches 6 lines at the hinder end, gradually decreases to 1 inch 9 lines at 

 the fore part. A few teeth have been cemented to the alveolar plate in the anterior 

 fragments, and perhaps in the places near which they were found, for numerous 

 scattered teeth of the Ichthi/osaitrus campyhdon, and doubtless of the same individual as 

 the jaws, were exposed in the fragments of the Chalk rock containing those parts. 



The outer surface of the dentary bone, PI. 4, fig. 1, is convex, the inner surface 

 at the part where the second joins the third fragment, about 1 foot 6 inches from the 

 anterior end, is divided into two longitudinal channels by the base of the inner alveolar 

 wall; which base is perforated lengthwise by the dental canal. As we trace this part of 

 the dentary backwards, the alveolar groove progressively shallows and diminishes, and 

 the lower groove widens and increases ; the section of the dentary at the back part of 

 this fragment, two feet from the fore end of the whole portion of the bone preserved, 

 presenting a sort of hour-glass form, the upper and under portion being connected by 

 a very thin plate. The form of the section displayed -at the fractured end is given in 

 PI. 4, fig. 1*. The coarser central osseous texture appears to have been included 

 within a thin, dense, exterior crust, about a line in thickness, and the same crust 

 suri'ounds the canal c. The outer surface of the dentary piece shows a shallow groove 

 about two thirds of an inch below the outer alveolar border, into which groove the 

 several vascular foramina open which are continued from the canal, fig. I c. 



The portion of the right ramus of the same lower jaw, PI. 4, fig. 2, includes 

 the termination of the splenial pieces with the commencement of the symphysis, 

 and includes an extent of the dentary piece, 32, measuring one foot three inches in 

 length. The vertical diameter of the dentary at the hinder fractured end, fig. 2*, is 

 three inches, and at the front end, fig. 2**, is two inches two lines. The inner alveolar 

 plate is sent off about an inch below the upper border of the thick outer plate ; and 

 forms the floor of the groove before it rises to form the inner wall ; it slightly increases 

 in thickness in forming the rounded border of that wall ; the diameter of the floor of 

 the socket is three lines : the depth of the alveolar groove is two inches three lines, its 

 breadth is ten lines and a half. Portions of both splenial bones, somewhat dislocated, 

 shown at 31, 31. 



The cavity in the dentary beneath the alveolar wall is reduced to a mere groove 

 midway between the fractured ends, with the exception of whicli the whole of the now 

 flattened inner surface of the dentary is in contact with its fellow, forming the strong 

 and long " symphysis raenti." At the fore part of the fragment the alveolar groove is 

 reduced to a depth of eleven lines, and a breadth at its outlet of nine lines. One of 

 the transverse canals is exposed at the anterior fracture, which passes from the inner 

 longitudinal canal to the outer groove. Wherever the bone is broken, that modification 



