258 MESSRS. HANCOCK AND HOWSE 



rods the component parts of an endo-skeleton, or do they par- 

 tially belong to an exo-skeleton ? 



At first sight one is disposed to look upon these plates and 

 rods as the remains of an endo-skeleton in a more or less dis- 

 turbed condition ; but on a closer examination a certain difficulty 

 presents itself. There is an element or two too much, that are 

 not found in the usual ichthyic skeleton. The sigmoidal plates 

 and their rods (fig. l,j,f, i) are additional parts that are very 

 perplexing ; had they not existed, there would have been no 

 difficulty in the matter. The lozenge-shaped areas and the late- 

 ral plates in that case might have been assumed at once to be a 

 partially ossified vertebral column with the neural and haemal 

 spines attached, and the hour-glass-shaped plates the fin-sup- 

 ports. But these redundant sigmoidal plates and rods must 

 likewise be taken into account. What are they ? 



We have to express our indebtedness to Prof. Huxley for an 

 answer to this question. 



All the Pycnodonts have supplementary spinous processes, 

 which extend from the vertebral spines to the interspinous pro- 

 cesses or fin-supports. The lateral rods, then, in Dorypterus 

 would appear to be the homologues of these supplementary spi- 

 nous processes. Such being the case, the only remaining ano- 

 malous feature to be explained is the sigmoidal plates. These, 

 there can be little doubt, are dermal, and consequently do not 

 belong to the endo-skeleton ; they are parts of an exo-skeleton, 

 and therefore belong to the same category as the ventral plates, 

 which are assuredly dermal. But this is not so clear with re- 

 gard to the peculiar compound columns with which the latter 

 are connected, though it would seem evident that the lower por- 

 tion, which is continuous with the plates, should be so consi- 

 dered. The upper and larger portion, however, of the column, 

 which is composed of numerous elongated rods articulated 

 lengthwise with each other and with the upper prolongation of 

 the plates, probably belong to the endo-skeleton. These pecu- 

 liar columns, in their compound structure and twisted appear- 

 ance, closely resemble similar columns, which are considered to 



