70 FOSSIL REPTILIA OF THE 



plate descends eight lines lower than the outer one, and the outlet of the alveolar 

 groove has a corresponding oblique aspect downwards, and a little outwards : the 

 breadth of the groove at the outlet is 1 inch 2 lines : the depth of the groove, to the 

 inner border, is 1 inch 6 lines : the breadth of the alveolar part of the pre maxillary, 

 including the plates, is 2 inches and a half. At the anterior fractured end of the 

 left premaxillary, (T. XXVI, fig. 2, 22,) ten inches in advance of the hinder fracture, 

 the vertical diameter of the bone is 2 inches 10 lines, the breadth of the lower alveolar 

 part is 1 inch 9 lines, the depth of the alveolar groove is 1 inch 5 lines, the breadth 

 of its outlet 1 inch 1 line. Here the two premaxillaries are in contact at the upper 

 borders, which have progressively increased, after overlapping the nasals, to a thickness 

 of 7 lines, The inner alveolar plate is sent off about half an inch below the upper 

 border, extending inwards and downwards, and dividing the nasal from the alveolar 

 groove, then descending, in contact with the same plate of the opposite premaxillary, 

 for about an inch of vertical extent : the thickness of the plate near its origin is 3 lines, 

 whence it increases to 7 lines at its lower rounded border, ib. at' . The elliptical area 

 of a canal, 4 lines in diameter, o, is exposed above the origin of the inner alveolar plate. 

 The narrow exterior groove, g, sinks 3 lines into the substance of the bone, and slightly 

 expands towards its bottom. The outer groove and the inner canal communicate by 

 transverse anastomosing channels at certain parts. 



The whole of the upper surface of the premaxillaries forms a smooth arch of bone, 

 describing in transverse section a semicircle, and impressed only by the longitudinal 

 groove each side, for the lodgement of a vessel on and probably also a branch of the 

 fifth pair of nerves. 



The portion of the lower jaw consists of the dentary and splenial pieces,* both 

 (hslocated, the former slighly. At the back part of the left ramus, (T. XXVI, fig. 1,) 

 the lower border of the splenial, (ib. 31,) has been pressed inwards and downwards 

 from that of the dentary, (ib. 32,) and slightly rotated so as to incline its inner vertical 

 wall outwards, where it is pushed into the groove or concavity of the dentary, which 

 it naturally closes, applying itself to the side of the inner alveolar plate of the dentary. 

 The right splenial, (ib. 31,) has been still more displaced, its lower border being pushed 

 against the base of the inner alveolar plate of the left ramus. 



Both splenials are exposed at the anterior fracture of the rami, (T. XXVI, fig. 2,) 

 six inches in advance of the preceding, the right being here, also, above the left, and 

 removed from its own ramus to contact with the base of the inner alveolar plate of the 

 left ramus. The vertical diameter of the splenial, which is two inches at the hinder 

 fractured part, has diminished to little more than one inch at a distance of five inches 



* Sec the Cut, fig. 9, p. 17, of the ' Monograph/ Part ii, CrocodUia and Ophidla, of the London Clay, 

 in which the different pieces of the complex lower jaw of Reptiles is figured : and where 29 is the 

 "articular," 29' the "surangular," 30 the "angular," 31 the "splenial," 3l' the " coiuplemental," 32 the 

 " dentary," in the Alligator. 



