OF THE PELYCOSAURIAN GENUS DIMETROIX)N. , 21 



that they did not articulate with the pterygoids on their inner side opposite the 

 external processes, as at first supposed, but much further back. It is probable that 

 there was a large mass of cartilage between the basipterygoid processes and the ptery- 

 goid comparable to the meniacns ptrri/i/oideiis described by Howse and Swinnerton in 

 the developing Sphenodon skull. 



The parasphenoid : From between the basipterygoid process extends anteriorly a 

 vertical, compressed plate (Fig. 2, PI. VII, and Figs. 4 and 5, PI. V) which extends 

 directly upAvard in the median line of the skull. The point of union of this plate 

 and the basisphenoid is marked on the upper edge by a deep notch. It has been 

 shown by Parker, Siebenrock, Howse and Swinnerton and others that the basi- 

 sphenoid of the adult reptiles is a compound bone formed of the true cartilaginous 

 basisphenoid and a dermal ossification which is the parasphenoid of the amphibians. 

 In embryonic and even in early postembryonic life in Sphenodon (according to Sieben- 

 rock) the suture between the two is traceable. In the forms with a cartilaginous inter- 

 orbital septum {Crocodilia, Lacertilia and Chelonia) the cartilaginous presphenoid is 

 not ossified and the parasphenoid extends as a slender styliform process from the 

 anterior end of the basisphenoid beneath the cartilaginous interorbital septum and 

 supports in embryonic life the membranous floor of the pituitary space. There is no 

 doubt that the anterior process of the basisphenoid in the Pelycosauria, as in the 

 Lacertilia and Rhyncocephalia vera, is the remnant of the parasphenoid united to the 

 basisphenoid and not the presphenoid as first described by Baur and Case ('99). 



The ethmoid: Instead, how'ever, of the parasphenoid process of the Pelycosauria 

 ending as a slender rod in the floor of the pituitary space it extends upward as a strong 

 slender plate and unites above with a second plate which is in contact with the lower 

 surface of the frontal bones. The suture between the parasphenoid and this plate is 

 closed but its position is marked by a low ridge showing the point of coosification. 

 The upper edge of the upper plate is planted firmly against the under side of the 

 frontals and there seems to be ample evidence of a direct sutural union but as the 

 region is somewhat crushed it is possible that the plate did not quite touch the frontal 

 in life but was connected with it by cartilage and that it has been forced into close 

 contact by the accidents of fossilization ; however it may be, the relations of the bone 

 would not be altered. The anterior edge of the plate is irregular and very thin show- 

 ing that it passed gradually into the cartilage of the interorbital septum in front. The 

 upper portion of the posterior edge is thin but the inferior posterior angle is thickened 

 and rounded, there is a deep notch between this angle and the parasphenoid below 

 and this notch marks the position of the escape of the second pair of cranial nerves. 

 There is no trace of either orbito- or ali-sphenoid ossification, as remarked above. 



A. p. S.— XXI. c. 



