1 94 Messrs. Hancock & Atthey on Dipterus and Ctenodus, 



bones. And the surface of the wliole, Avhich is in a good state, 

 is devoid of ornamentation, save that it is minutelj and 

 irregularly granular and punctate ; but the species vaiy a little 

 in these respects. 



In Mr. Atthey's collection of separate cranial bones there 

 are, besides specimens of the above, the median occipital of 

 three or four other species, all varying a little in contour and 

 in surface-character ; so that we have here additional coito- " 

 borative evidence that our shales contain six or seven species 

 of Ctenodus. 



The opercula likewise resemble those of Dipterus • they are 

 large, stout, slightly convex, irregularly circular plates, Avith 

 one side of the margin a little flattened, and slightly produced 

 at each end of the flattened space ; the surface is punctate and 

 granular like the cranial bones. We possess six or seven 

 different forms of these gill-covers, two of which have been 

 identified as belonging to C. elegans and C. ohliquus respec- 

 tively. But, for a particular description of the various oper- 

 cula, we must refer to our paj)er in the 'Annals' previously 

 quoted, merely observing 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 w^ell 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 Dipterus. The tail-fin is 

 heterocercal and rhomboidal ; and the aniil 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 justify 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, 

 Avhich, though we are not able to determine the fact, we be- 

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



• In Lepidodren the shoulder-girdle seems to be reduced to a single 

 laicge Lone h.aving considerable resemblance to the above. 



