Parasphenoid of a Faheom'scoicL 4.20 



the parasphenoid apj)ears as a more or less slender shaft or 

 process in front of the basisphenoid. For the Ichtiij^osaiiria 

 the para-basisphenoid of Oph'J/almosaurus (5) may be taken 

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



Thus there is a charac-teristic form amongst the Reptilia 

 for the compound para-bnsisphenoid 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 Ampliibia are mucli too specialised to be of 

 much assistance. All the Atiura possess a characteri.^tic 

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

 tiie T directed posteriorly. There is no trace apparently 

 of any basisphenoid. 



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

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

 Lvxomma, 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-occip:ta!,and wliatis 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 Opldhalmosaurus, certain 

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

 '' the sides of the basisphenoid slope upwards, passing 

 imperceptibly into the opisthotic and pro-otic. On eacii 

 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 imf-ressel on the bone on the inner side 

 of each of these processes is a groove which leads round 

 from the side of tiie 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 tiie 

 reptiles quoted above : hence the general form of this bone 

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

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

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

 numerous bones in the Branchiosauridse, &c., of the same 

 g.neral 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 Meijahchthys, says : " the bas!Sj)henoid of Megalickthys 

 has sometimes carotid foramina just as in Loxomma. It has 



Ann. ds Maj. N. Hist. Ser. 8. Vol. xvi. 30 



