ME. T. W. BRIDGE OR THE OSTEOLOGY OP POLYODON POL1UM. 
C97 
Both in the young Tadpole and in the very early stages of the developmental history 
of the Salmon, the chondrification of the roof of the periotic capsule is incomplete 
in the region of the primitive involution ; and it seems conceivable that in Polyodon 
this primitive fenestration has been retained in the a.dult, while the atrophy ot the 
inner or cranial wall of the capsule has caused it to communicate with the interior of 
the cranium. 
Though the last view of the nature of these fen es tree seems to me to be the more 
probable one, yet it is possible that neither is correct, and that they may be of 
secondary origin — the result of local absorption of the cartilage of the otic capsule. 
But if my view be correct, then the existence of these parietal foramina in Polyodon 
is an important anatomical feature in which that Fish differs from all other Ganoids 
and Teleostei, and approaches the Plagiostome Elasmobranchii. Mr. Parker informs 
me that he has discovered similar perforations of the otic cartilage beneath the 
posterior canal in the Urodele Amphibian, Siren lacertina, to which he attaches a 
like significance. This fact is an important addition to the list of anatomical 
characters common to the Ganoids, Elasmobranchs and Amphibia. 
I may add that in Acipenser there is a depression in the roof of the otic cartilage 
external to the posterior vertical semicircular canal, and at the bottom of it there is a 
fat-infiltrated canal which penetrates for a short distance into the substance of the 
cartilage, but does not communicate with the cranial cavity. The enormous growth 
of the cranial cartilage, which has effectually masked other embryonic features in the 
cranial structure of the Sturgeon, has probably obliterated the primitive communication 
of these canals with the cranial cavity. 
The oblique lateral ridges and decurrent wing-like processes described in Polyodon 
are also represented in the Sturgeon. Behind the auditory organs, “ and separated 
from them by a deep lateral fossa, are two wing-like processes, which are directed 
outwards and obliquely backwards, and proceed not from the walls of the cranium 
proper but from those of the spinal column, where it joins the skull.”' 1 ' This ridge, 
however, runs from above downwards and backwards, the foramina for anterior roots 
lying above the ridge, and those for posterior roots below it. 
The second and smaller foramen (figs. 3 and 4, x.) in the anterior part of the /-shaped 
groove lies at the bottom of a deep basin-shaped depression in the roof of the otic 
cartilage, and commuuicates through a short canal with an elongated slit-like opening 
(x.) on the infero-lateral face of the periotic capsule, between the articular groove for 
the hyomandibular, and the attachment of the metapterygoid ligament. In the fresh 
specimen it was seen that the slit-like inferior opening was situated in the inner 
wall of the spiracular passage, and that the mucous membrane of the pharynx was 
continued through it into the canal, but the upper part of the latter and the 
depression into which it opened above, as well as the jf-shapecl groove, were filled with 
connective tissue infiltrated with fat. It may, in fact, be said that the first visceral 
* Huxley, Z< c. cit. 
4 u 2 
