634 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 22, 
black matter continuous with that which covers the plates, and which 
seems to be the remains of the ruptured film or dermal envelope. In 
fact, in one or two places where the plates are more approximated 
than usual, the continuity of the connecting film is quite obvious. 
Now comes the question, Are the whole of these plates and rods 
the component parts of an endo-skeleton, or do they partially 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 disturbed 
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. 1, 7, 7',7) 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 lateral plates in that 
ease might have been assumed at once to be a partially ossified 
vertebral column with the neural and hemal spines attached, and 
the hour-glass-shaped plates the fin-supports. But these redun- 
dant 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 Pyenodonts have supplementary spinous processes, which 
extend from the vertebral spines to the interspinous processes or 
fin-supports. The lateral rods, then, in Dorypterus would appear 
to be the homologues of these supplementary spinous processes. 
Such being the case, the only remaining anomalous 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 regard to the peculiar compound columns with 
which the latter are connected, though it would seem evident that 
the lower portion, which is continuous with the plates, should be so 
considered. The upper and larger portion, however, of the column, 
which is composed of numerous elongated rods articulated length- 
wise with each other and with the upper prolongation of the plates, 
probably belongs to the endo-skeleton. These peculiar columns, in 
their compound structure and twisted appearance, closely resemble 
similar columns, which are considered to be ribs, in Pycnodus rhombus, 
Ag., and in which they are apparently joined to the vertebra. In 
Dorypterus, too, the compound columns seem to have their upper ex- 
tremities united to the lateral plates or neural spines. It would 
therefore seem probable thatin 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, according to 
Agassiz’s restoration, also Platysomus ; but in these it seems to be 
an enlarged development of an interhemal spinous process; while 
in Dorypterus, in which there appear to be two such rods, they seem 
