CRETACEOUS ICHTHYOSAURS. 229 



bones, including at the back part about three inches of the exposed, and apparently 

 pointed terminations of the nasal bones (PI. 2, fig. 2, 15.) These, however, extend 

 much further forwards than they appear to do externally, their anterior ends being 

 overlapped by the premaxillaries, 22. The breadth of the premaxillaries at the 

 fractured hinder end of this specimen is 5^ inches, at the distance of one foot from 

 that fractured end it is 3| inches, and the decrease seems to have been rather more 

 gradual in advance of this part. The total length of the jaws from the point of union 

 of the premaxillaries above the nasals, may therefore be estimated at about three feet. 



The breadth of each nasal, where they dip beneath the premaxillaries, is one inch 

 three lines : the upper surface presents a longitudinal furrow midway between the 

 margins of the bone, into which furrow a longitudinal ridge at the under surface of the 

 premaxillary fits, thus strengthening the union between the two bones. The nasal 

 bone forms a parallel ridge, or angular projection, from its own under surface, which 

 divides the inferior concavity into two parts, the median and broader concavity being 

 somewhat angular in form. The actual pointed ends of the nasals are visible at a 

 fractured surface, (PI. 3, fig. 2, 15,) nine inches in advance of the point where 

 they are concealed by the median junction of the premaxillaries : their section here 

 presents the form of a curved lamina of bone, thickest at its median border, and half 

 an inch in breadth, and this may be traced beneath the fractured portion of the 

 premaxillary three inches further forwards. 



The breadth of the nasal cavity at the back part of the fractured end, (PI. 3, 

 fig. 1,) is rather more than two inches : at the anterior fracture, fig. 2, it is reduced to 

 ten lines. 



The median borders of the premaxillaries, (PI. 3, fig. 1, 22,) before their junction 

 above the nasals, (ib. 15,) are about one line thick, and the bone increases to a 

 thickness of three lines, above the part where it sends off, inwards and downwards, 

 the inner alveolar border, (ib. al,) which is at a distance of an inch and three fourths 

 from the upper median border. On the outside, opposite the origin of the inner 

 alveolar plate, the premaxillary is traversed by a straight longitudinal groove, (PI. 2, 

 fig. l,y,) four lines in breadth, which contracts, as it advances forwards. The outer 

 alveolar plate, (PI. 3, fig. 1, «,) increases in thickness to six lines, and terminates 

 below in a convex border. The inner alveolar plate, (ib. al,) forms the chief part of the 

 arched roof of the upper dental groove, and has there scarcely a line in thickness ; but 

 as it descends, it rapidly gains a thickness of five lines at its inferior convex border. 

 There is a slight solution of continuity between the arched and descending portions of 

 the inner alveolar plate, (ib. al',) at the hinder fractured end of the specimen, and the 

 descending plates might at first sight be taken for the palatine bones ; but these, in 

 other Icldliyosauri, are vertical plates, which lie parallel with, and on the inner side of 

 the descending alveolar plate of the premaxillary, and do not reach so far forwards as 

 where the nasals are wholly overlapped by the premaxillaries. The inner alveolar 



