THE OSTEOLOGY OF THE DIADECTIDAE 157 



rock, 1897) that in Sphenodon the rostrum of the basisphenoid is the 

 parasphenoid; the basisphenoid with its anterior rostrum develops 

 from three centers — two posterior, in the cartilage of the skull axis, 

 and one anterior, in the lining membrane of the floor of the pituitary 

 space. The same observation has been reported and confirmed in 

 the development of the Lacertilian skull. It is evident, then, that 

 the rostrum of the basisphenoid is the remnant of the parasphenoid 

 in the reptiles. (It is common to refer to this rostrum as the pre- 

 sphenoid — a distinct error, as the presphenoid is a continuation of 

 the cartilaginous basicranial axis, and not a membrane bone.) 



There is no such rostrum developed in the turtles. I am aware 

 that Parker reported (Parker and Bettany, 1877, p. 214) that the 

 basisphenoid of the turtle is developed from three centers, as in the 

 Lacertilia, but this is denied by Siebenrock (Siebenrock, 1897), who 

 also cites Rathke (Rathke, 1848), as follows: 



Dass sich das Basisphenoideum bei den Schildkroten nur in einfacher Zahl 

 bildet denn selbst bei denn reifern Embryonen, konnte Rathke nicht das geringste 

 Zeichen anfinden, dass es urspriinglich aus einem hinteren und vordern Stiicke 

 bestanden hatte. 



Siebenrock further explains the apparent rostrum of the turtles by 

 the elongation and approximation of the trabeculae inferiores 

 (Siebenrock, 1897, p. 18). 



In an examination of the Reptilia I find the following condition: 

 The parasphenoid is absent as a rostrum of the basisphenoid in the 

 Chelydosauria, Testudinata, Cotylosauria partim (Telerpeton, Pareia- 

 saurius ( ?), Procolophon). It is present in the Ichthyo pterygia, Saurop- 

 terygia, Squamata, Theropodous Dinosaurs (Diplodocus), Cotylo- 

 sauria partim (Pariotichidae, Labidosaurus). 



It now becomes necessary to discuss this point in connection with 

 the fifth point : 



5. The absence of the prevomers and the presence of an anteriorly 

 placed single vomer. In the forms where there is a rostrum on the 

 basisphenoid there are always paired prevomers, but where this ros- 

 trum is not developed there is a single, anteriorly placed vomer and 

 no prevomers. 



Broom has shown (Broom, 1904) that the median vomer of the 

 turtles is probably the parasphenoid. The condition of the vomer of 



