I08 NEW YORK STATE MUSEUM 



canal systems ; (2) discreteness of the central elements and their separation 

 on either side of the middle line by the median occipital ; (3) reduced num- 

 ber of median series of plates ; and (4) absence of any evidence of articula- 

 tion or other connection between the head shield and dorsal body plates. 



We have next to speak of the configuration of the basis cranii, by 

 which term is understood, of course, not the inner surface of the cranial 

 buckler, but the floor and posterior wall of the chondrocranium, the passage- 

 way provided for the initial portion of the vertebral axis, ossified para- 

 chordal cartilages, and the greatly elongated parasphenoid which forms the 

 roof of the mouth. In the first place it should be noted that a bony exten- 

 sion of the external occipitals is developed along the posterior margin of 

 the head shield in the form of a thin lamina or septum which extends 

 nearly vertically downwards into the soft parts until reaching the level of 

 the parasphenoid, with whose hinder extremity it joins. It is along the 

 plane of this septum that the exoccipito-central canals penetrate downward 

 and inward from the outer surface by means of funnel-shaped openings, the 

 interior of which is filled with cancellated bony tissues. The function of 

 the posterior septum seems to have been to impart rigidity to the arch of 

 the head shield, and to serve as a partial support for the parasphenoid 



The last named element resembles in a general way the familiar 

 lozenge-shaped bone in Dipterus and modern Dipnoans, but is remarkable 

 for its great expansion in front, where it occupies nearly the entire width 

 of the head shield. Becoming narrower in the occipital region, it is con- 

 tinued backward as far as the posterior margin in the form of an arched 

 laminar plate, not unlike that of Neoceratodus in form, upon which rests 

 the parachordal cartilages and notochordal sheath. This hinder portion of 

 the parasphenoid was interpreted by Cope as consisting of a pair of distinct 

 elements, called by him the ' lateral alae of the axis," or, in another place, 

 " descending osseous laminae " , but it is clear from well preserved speci- 

 mens that only a single ossified element is attached to the floor of the carti- 

 laginous cranium. A right understanding of this feature shows that in the 

 form under discussion the parasphenoid is produced posteriorly to the same 



