SXRL'CTUKE OF CEETAIN PALEOZOIC DIPNOI. 
197 
Ctenodus, Sagenodus, and Ceratodus. 
The close correspondence whicb has been traced in the structure of these 
three genera, notably as regards the palate, the lower jaw and the shoulder- 
girdle, leaves no doubt as to their near affinity. The structure o£ the skull- 
roof, i£ followed through the same three fishes, shows less constancy; on the 
Ctenodus"? A. Cleitlirmn. B, C. Clavicle, upper and lower surfaoea. 
Both bones are represented by numej-oiis specimens in the Atthey Collection. 
other hand, it shows an interesting progressive change, starting from the 
pattern first established in Dipterus platyceplialus. The change consists in 
the expansion, at the expense of the surrounding bones, of the" median 
"parietal" and interfrontal, together with the reduction of the bones of the 
snout region (nasals and prenasals). From Dipterus to Ctenodus the change, 
though obvious, is not great. From Ctenodus to Sagenodus it is in both 
respects very marked ; in Ceratodus it has been carried to an extreme, for 
