Parasphenoid of a Paheoniscoid. 429 



the parasphenoid appears as a more or less slender shaft or 

 process in front of the basisphenoid. For the Ichthyosauria 

 the para-basi sphenoid of Ophthalmosaurus (5) may be taken 

 as a type, and for the Sauropterygia that of Tricleidus (5). 



Thus there is a characteristic form amongst the Reptilia 

 for the compound para-basisphenoid bone, compound as we 

 know from a study of fossil and recent types. 



Coming now to a consideration of the Amphibia, we find 

 that the living Amphibia are much too specialised to be of 

 much assistance. All the Anura possess a characteristic 

 T-shaped bone termed the parasphenoid, with the cross of 

 the T directed posteriorly. There is no trace apparently 

 of any basisphenoid. 



When we turn to the fossil Amphibia, we find a much 

 more satisfactory state of affairs. Watson (6), speaking of 

 Loxomma, says : " the basisphenoid, parasphenoid, and 

 ethmoid are fused together and only their lower surface is 

 well seen. The back of the basisphenoid is recessed for the 

 anterior end of the basi-occipital, and what is presumably the 

 posterior end of the parasphenoid projects backwards covering 

 the lower surface of the latter bone/' This backward pro- 

 jection of the parasphenoid (though not to underlie the basi- 

 occipital) is exactly paralleled in Ophthalmosaurus, certain 

 of the Sauropterygii, and even in Sphenodon itself. Further, 

 u the sides of the basisphenoid slope upwards, passing 

 imperceptibly into the opisthotic and pro-otic. On each 

 side of the bone in the region of the pituitary fossa is a 

 powerful process, the processus basipterygoideus, which has 

 a well-marked, smooth, articulating surface on its antero- 

 lateral side: deeply impressed on the bone on the inner side 

 of each of these processes is a groove which leads round 

 from the side of the basis cranii to open in front into a 

 foramen passing through the bone, which is undoubtedly the 

 carotid foramen/' Anteriorly to this the bone takes the 

 form of a narrow pointed splint exactly like those of the 

 reptiles quoted above : hence the general form of this bone 

 in Loxomma is exactly like that to be found in the reptiles. 

 Pleroplax, in the same paper by Watson, is stated to have a 

 very similar basis cranii. Finally, Fritsch (7) fioures 

 numerous bones in the Branchiosaundse, &c, of the same 

 general form as the compound para-basisphenoid of the 

 snake, and to these applies the term " parasphenoid." 



Coming now to the fishes themselves, and the primitive 



Teleostomes in particular, we find that Watson (6), speaking 



of Meyal'chthys, says : " the basisphenoid of Megalichthys 



has sometimes carotid foramina just as in Loxomma. It has 



Ann. & Maj. N. Hist. Ser. 8. Vol, xvi. 30 



