RTPSODON ETC. FROM BRITISH CRETACEOUS STRATA. 517 



obtained with this specimen, and are represented upon Plate XXII. 

 fig. 4. Although somewhat broken and compressed, they are suffi- 

 ciently well preserved to show that the walls of the brain -case were 

 completely ossified. The sutures are so obscure that the determi- 

 nation of the homologies of the various parts is no easy task ; however, 

 some of the elements can be identified with more or less certainty. 

 There can be no doubt that the fragment marked hm is the proximal 

 portion of the hyomandibular ; and the bone lying in front of it, 

 and to which it is attached (sp.oi), can only be the postfrontal (sphe- 

 notic). The upper part of this is broken. The next bone below 

 (pr.ot) is perforated anteriorly by a large foramen (probably for the 

 5th nerve), and occupies the position of the prootic, being wedged 

 in between the bones which are believed to be the basisphenoid, 

 alisphenoid, postfrontal, and opisthotic ? Attached to the hinder 

 part of the prootic there is a portion of a bone (op.oi) which appears 

 to be either the opisthotic or eccoccipitcd ; but as all the parts behind 

 this are wanting, its true homology cannot be determined. If this fish 

 possessed an opisthotic, then the bone in question is that otic ele- 

 ment ; but if, as Prof. Cope thinks, Portheus had no opisthotic, then 

 this bone must be the exoccipital. The basioccipital and para sphenoid 

 are wanting. The bone marked bs seems to be one side of the upper 

 forked part of the basisphenoid, occupying, as it does, precisely the 

 position of the Y-shaped bone in the pike, which Prof. Huxley has 

 referred to the basisphenoid (' Elements of Comparative Anatomy/ 

 1864, p. 169). Following upwards from the front part of this bone 

 there is another (as), which, from its relations to the surrounding 

 elements, is most probably the alisphenoid. The anterior part of 

 the basis cranii is formed by the bone marked ps, which occupies the 

 position of, and probably represents, both the presphenoid and orbito- 

 sphenoids. Seen from below, this bone is broad posteriorly and 

 narrowed anteriorly, with a median rugose, pear-shaped convexity 

 upon the under surface. From the sides are given off broad wing- 

 like expansions, which pass upwards to meet the frontal bone. These 

 wings sweep round anteriorly, so that they meet in the middle line. 

 The bone (fr) in front and above the one last mentioned is probably 

 a portion of the frontal ; from the outer side of this, and just above 

 what must have been the orbit, there is a downward process, having 

 upon it a bone with a rugose surface, o b, which it is thought may, 

 perhaps, be a supraorbital, but is most probably one of the hinder- 

 most of the series of bones which surrounded the orbit. The form 

 of the upper part of the frontal seems to indicate that this region of 

 the skull was raised into a prominent crest ; and this is a character 

 which Prof. Cope has shown to exist in other species of the genus. 



Whether the bone in the region of the letters pa is a portion of 

 the parietal or not it is impossible to say; but certain it is that 

 immediately below the letters this bone is suturally united with the 

 one which has been called alisphenoid. 



The sclerotic is well ossified, and parts of four or five plates are 

 preserved ; but neither of them is sufficiently perfect to show its 

 precise form. 



