ON DORYPTERUS HOFFMANNI. 259 



be ribs, in Pycnodus rhombus, Ag., and in wbicb tbey are ap- 

 parently joined to tbe vertebras. In Dorypterus, too, tbe com- 

 pound columns seem to have their upper extremities united to 

 the lateral plates or neural spines. It would therefore seem 

 probable that in this genus the upper portions at least of the 

 columns are likewise ribs. 



The great abdominal rods have a strong resemblance to the 

 bone similarly situated in such fishes as Vomer, Zeus, and, ac- 

 cording to Agassiz's restoration, also Platysomus ; but in these it 

 seems to be an enlarged development of an interhasmal spinous 

 process ; while in Dorypterus, in which there appear to be two 

 such rods, they seem to be developed in connexion with the 

 posterior pair of ventral plates ; nevertheless they are probably 

 internal bones. 



On the whole, then, it seems pretty clear that these peculiar 

 plates and rods in Dorypterus are the component parts of both 

 an endo- and an exo-skeleton. The lozenge-shaped plates or 

 areas are the vertebral centres, or rather the remains of them ; 

 the lateral plates are the neural and hasmal spines, and the rods 

 in connexion with them are the supplementary spinous pro- 

 cesses ; while the hour-glass-shaped plates are the interspinal 

 processes or fin-supports, and the greater portion of the com- 

 pound thoracic columns are apparently ribs. All these, as well 

 as the great abdominal rods, are component parts of the internal 

 bony skeleton. 



The sigmoidal and ventral or thoracic plates, together with 

 the upper prolongation of the latter, forming the basal portion 

 of the compound thoracic columns, seem to be developments of 

 the skin, and consequently belong to an exo-skeleton. The 

 plates, too, on the shoulder, that apparently correspond to the 

 sigmoidal plates of the posterior portion of the body, are most 

 likely also dermal ; and if so, the ridges in their transverse cen- 

 tre which form a line extending from the nape to some distance 

 down the body are probably true mucus-tubes, which they so 

 much resemble. 



There is not much more to say with respect to this matter ; 

 we may observe, however, that in Dorypterus the ganoid type 



