96 FOSSIL REPTILIA OF THE 



in the Crocodiles, which elements they resemble in their small relative size, but are of more 

 simple structure. Each is feebly and regularly bent, the convexity (in the petrified skull) 

 turned towards each other, converging forward to their junction with the small flattened 

 basihyal, which junction seems to have been by much ligamentous tissue. I noted in a 

 fossil cranium of a full-sized Ichthyosaurus communis that the hind ends were 4f inches 

 apart while the fore ends were but \\ inch apart. The total length of the bone was 

 4 inches 2 lines ; the breadth of the hind end was \ an inch, that of the mid-part 5 lines. 

 In the Ich. lonchiodon (PI. XXXI, fig. 5) each thyrohyal is a fifth part the length of the 

 mandible. The stylohyals appear to have retained their fibro-cartilaginous or cartila- 

 ginous tissue, and have consequently disappeared. 



The presphenoid (PL XXV, fig. 1, 9) is a long, slender, trihedral bone, broadest where 

 it joins, and commonly coalesces with, the basisphenoid ( 5 ) and, along the narrower part, 

 with the two lower sides, converging to a median obtuse angle. It divides the long and 

 narrow, pear-shaped interpterygoid vacuities (s,s). 



Of the orbitosphenoids I have no exact knowledge ; they may not have been 

 ossified, there is no trace or sign of the lacertian columellar bone near that part of the 

 cranium. 



The frontals, or midfrontals (Pis. XXVII, fig. 1, 11), are small, transversely convex in 

 greater or less degree according to the species, curving in toward the mid-suture in most, 

 and bending outward and downward to the obtuse angle intervening between the nasal 

 (15) and the postfrontals (12). At the hind end of the frontal suture each border curves 

 outwardly to contribute their large share to the 'foramen parietale,' then converging and, 

 in some species (Ich. latifrons, PI. XXVII), wholly encompassing it ; here, also, as in some 

 other species, the midfrontals distinctly join the postfrontals. In all the species the hind 

 border joins the parietal (7), the fore border the nasal (15). The midfrontals are widely 

 separated from the orbits by the postfrontals and nasals. I have not had clear evidence 

 of their touching the prefrontals, or of the presence of distinct superorbitals. In 

 Ichthyosaurus latifrons (PI. XXVII, fig. 1) the frontal suture becomes obliterated, and 

 the bone (11) is convex lengthwise as well as across. 



The postfrontals (ib., and PI. XXIII, fig. 1,12) exceed the midfrontals in size. Each 

 extends from a mesial angle outwards, expanding horizontally, and inclining to form its 

 share of the superorbital border ; thence the postfrontal is continued backward, bending 

 down to join the postorbital (12') and prosquamosal (27'), and contracting to a notched and 

 pointed end, which receives and overlaps the fore end of the zygomatic ray of the mas- 

 toid (8). The similarity of character in the postfrontal (12) and mastoid (8) is worthy 

 of note in regard to their general homology as cranial diapophyses. 



The haemal or pleurapophysial arch of the third or frontal segment of the cranium is 

 modified to constitute the lower jaw. 



In the mandible of Ichthyosaurus the articular element (PI. XXVI, fig. 2, 29) 1 is 

 1 Cuviee, ' Oss. Foss.,' v, pt. ii, p. 2/2, pi. xvi, figs. 4 and 5 (Varanus), d (' articulaire '). 



