174 IOWA GEOLOGICAL SURVEY 



below. It is along the plane of this septum that the exoccipito- 

 central canals penetrate obliquely downward from the outer 

 surface by means of funnel-shaped openings, the interior of 

 which is filled with cancellated bony tissue. The function of 

 the hinder transverse septum seems to have been to impart rigid- 

 ity to the arch of the headshield, and to serve as a partial sup- 

 port for the parasphenoid. The space included between the 

 two septa just described is that which Newberry designated as 

 a "cerebral" and Cope as a "nuchal chamber," both authors 

 apparently regarding it as closed along the sides as well as at 

 either end. It was, however, only partially enclosed, its middle 

 portion alone being floored by the parasphenoid, or backward 

 prolongation of that element. No known organ could have been 

 lodged in this partially enclosed space, and in all probability it 

 merely contained fatty matter. 



We have already had occasion to speak of the element called 

 by Cope the parasphenoid, and in our opinion correctly identified 

 by him as such. In form this very tenuous plate resembles in a 

 general way the familiar lozenge-shaped bone in Dipterus, 

 Ctenodus and modern Dipnoans, but it is remarkable for its 

 great expansion in front, where it occupies nearly the entire 

 width of the headshield. Becoming rapidly constricted in the 

 occipital region, it extends backward over the space separating 

 the two transverse septa already mentioned in the form of an 

 arched laminar plate, not unlike that of Neoceratodus in form, 

 and serves as a floor for the parachordal cartilages and noto- 

 chordal 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", and in another place, "descending 

 osseous laminae"; but it is clear from well preserved specimens 

 that only a single ossified element is concerned in flooring the car- 

 tilaginous cranium and projecting backward as far as the ex- 

 treme posterior margin. A right understanding of this feature 

 shows that in the form under discussion the parasphenoid is 

 produced posteriorly to the same extent as in Neoceratodus and 

 Lepidosiren ; hence Cope 's statement requires rectification when 

 it is said that the corresponding bone in modem forms is abnor- 



