22 



THE MORPHOLOGY OF THE SKULL 



A plate identical in position and relations with this one has recently (Broom, 

 '04) been demonstrated in Lystrosaurns (Ptychognathns), see Fig. 6. In the Cro- 

 eodilia, Lacertilia and Chelonia the interorbital septum is cartilaginous, and in the 

 Ophidia the osseous septum is formed in a very different manner, by the extension of 

 the brain case forward and the downward development of the frontal bones to meet 

 the parasphenoid without any intervening ossification of a median septum. 



In the young Sphenodon there is a very complete cartilaginous septum which is 

 double in the region of the nasal and oral capsules, but in the orbital region is single 

 and reaches upward toward the frontal, from the upper surface of the parasphenoid. 

 This plate is called by Howse and Swinnerton the presphenoid cartilage, but the 

 presphenoid is a basi-cranial bone, and in the chondrocranium is that portion of the 



Fig. 6. Median section of the skuU of Lystrosaurus ( Plychognathus) laliroslris Owen. After Broom, bo., basi-oocip- 

 ital ; Is., basi-sphenoid ; eth , ethmoid ; fr., frontal ; fm., foramen magnum ; «., nasal ; p., parietal ; pp., preparietal ; 

 pf., pineal foramen, pmx., premaxillary ; pt., pterygoid ; ro., vomer. 



cartilage anterior to the pituitary region. It is evident that the whole of the cartilage 

 called by Howse and Swinnerton the presphenoid cannot be true presphenoid, but that 

 the anterior portion at least must belong to the interorbital septum, the ethmoidal 

 complex. 



The developing chondrocranium of the different orders of reptiles is, in all the 

 essentials of the relationships of the parasphenoid bone and presphenoid and septal 

 cartilages, the same; so that it is evident that the median plate of the skull of the 

 Pelycosauria here described is an ossification of the median septum of the skull directly 

 connected below with the parasphenoid bone, i. e., the ethmoid. 



