ON DIPTEKUS AND CTENODUS. 403 



here that the largest . are upwards of six inches in diameter ; 

 the smallest, that of C. elegans, about five -sixteenths of an inch, 

 though detached specimens of this species measure as much as 

 nine -sixteenths of an inch ; all the others are very much larger 

 than this. 



The ribs are well ossified both in Dipterus and Ctenodus. 

 Those of the latter are frequently found associated with other 

 remains of this fish, many of them having been identified with 

 the various species ; they are well arched towards the proximal 

 extremity, which is considerably enlarged ; and the central 

 channel is quite small, the cylindrical wall of bone being very 

 thick ; the ossification of the ribs is, in fact, almost complete. 

 The largest ribs are from six to eight inches long. 



The fins, so far as they can be made out in C. elegans, are 

 arranged in the same manner as in Dipteras, The tail-fin is 

 heterocercal and rhomboidal ; and the anal and ventral can be 

 traced, immediately before the caudal. 



On the whole, then, it appears quite certain that these two 

 genera are very closely related, as generally considered ; but 

 at the same time the several differences already pointed out 

 would seem quite sufficient to justifiy their generic separation, 

 at least for the present. 



We may remark here that a peculiar hatchet-shaped or 

 wedge-formed bone belonging to Ctenodus has occurred, which, 

 though we are not able to determine the fact, we believe to 

 belong to the shoulder- girdle.* Several specimens have been 

 found, which are divisible into five or six species by the modifi- 

 cation in the form ; they are paired bones. Their general 

 character is that of a flattened elongated bone, with one end a 

 little expanded, arched slightly, and gradually thinned out to a 

 fine edge ; it narrows a little towards the other end : one of the 

 lateral margins is slightly thickened, and is somewhat convex ; 

 the opposite margin is a little concave. From the narrow 

 extremity a strong wide process is given off at right angles, and 

 extends considerably beyond the concave margin. 



* In Lepidosiren the shoulder-girdle seems to be reduced to a single large bone having 

 considerable resemblance to the above. 



