Miscellaneous. 895 



The parietal foramen is bounded by both parietals and frontals ; 

 its presence is n mark of Labyriiithodont and Lacertian affinities ; its 

 formation is hlvc that in lyuuiia and Rhijnt'hucephulus. 



The temporal fosste are bonnded above by tlie parietal internally, 

 by the mastoid and postfrontal externally ; they are of an oval form, 

 with the great end forward : in their relative size and backward posi- 

 tion they are more Crocodilian than Lacertian. 



In the Ivhthyusaurus commitnis I have connted seventeen sclerotic 

 plates, forming the fore part of the eyeball. In a well-preserved 

 example the jjnpillary or corneal vacuity, as bounded by these plates, 

 is of a full oval form, 1 i inch in long diameter ; the length of the 

 l)lates (or breadth of the frame) being from 8 to 10 lines. In the 

 same skull the long diameter of the orbit is 4 inches. The deep 

 position of the sclerotic circle in this cavity, showed how they had 

 sunk, by pressure of the external mud as the eyeball became col- 

 lapsed by escape of the humours in decomj)Osition. 



The sclerotic plates are of an irregular, elongate quadrate form : 

 the borders by which they reciprocally join each other by squamous 

 sutures, are the longest, and are irregular ; the hind border is about 

 half the length of tlie plate, and is very thin ; tlie front or corneal 

 border is thicker, shorter, and is straight. From this border each 

 plate extends outwards and backwards as a flat slightly expanding 

 scale for more than half its length ; it then suddenly bends back- 

 wards and inwards, defining and encasing the extreme periphery or 

 circumference of the eyeball, and indicating the extreme oblateness 

 cf the spheroid. 



Whenever the antecedent forms of an extinct genus of any class 

 are known, the characters of such genus should be compared with 

 those of its predecessors rather than with its successors or with exist- 

 ing forms, in reference to gaining an insight into its true affinities. 



We derive a truer conception of the affinities of the Ichthyosaurus 

 by comparison with the Labyrinthodonts and other Triassic reptiles, 

 and of the Plesiosaurus by comparison with the Muschelkalk jMa- 

 crotrachelians, than of either by comparison with modern Lacertians 

 and Crocodilians. 



It is commonly said that the Ichthyo- and the Plesiosaurus re- 

 semble more the Lizards in such and such characters, and m a less 

 degree the Crocodiles, as in such a character. 



The truer expression would be, that the Lizards, which are the 

 predominating form of Saurians at the present day, have retained 

 more of the osteological type of the Triassic and Oohtic reptiles, and 

 that the Crocodiles deviate further from them, or exhibit a more 

 modified or specialized structure. 



As the Plesiosaurus is most allied to, or may be figuratively said 

 to be derived from the Triassic Pistosaurns, so the Ichthyosaurus, 

 by its fluted and partially-folded teeth, their loose inijtlantation, the 

 retention of the postorbital and sujiersquamosal, and the exclusion 

 of the frontal from the orbits, may be said to be derived from the 

 Labyrinthodonts ; l)ut the modifications are now such as to obliterate 

 all trace of the Batrachian affinities. 



