63 
ing to PARKER there are, in Lepidosteus, distinct, primary, opisthotic 
and epiotic centers of ossification. The dermal canal bones of the 
region are thus wholly separate and distinct from the otic bones, as 
they are in Polypterus and in Amia, and must be the homologues of 
the extrascapulars of the latter fishes. 
In Polyodon there is, according to CoLLINGE (No. 12, p. 506, 512 
und 519) no complete supratemporal commissure, the commissure being 
represented, on each side of the head, by a short branch of the main 
lateral canal which traverses a so-called occipital series of canal bones 
lying “upon the surface” of the so-called posttemporal bone. This 
posttemporal bone adjoins, in front, the so-called dermo-sphenotic, 
which latter bone occupies, as I have already stated, a position corre- 
sponding, in its relations to the lateral canals, to the squamosal and 
dermo-postfrontal combined of Amia and Teleosts. The posttemporal 
bone of CoLLınge’s descriptions would thus seem to be the extra- 
scapular of my nomenclature, and not the suprascapular, which latter 
bone is the posttemporal of ordinary English terminology. The com- 
missure of Polyodon, under this supposition, would be the homologue 
of the supratemporal crosscommissures of Amia and Polypterus, but 
it is to be noted that the mesial end of the commissure is shown 
lying superficial to the hind end of the parietal; that is, in the posi- 
tion of the middle head line of pit organs in Amia. The sense organs 
of the canal in Polyodon are said to be innervated, so far as I can 
understand the descriptions, by a branch of the lateralis vagi, given 
off close to its base, and which is, apparently, the ramus supratem- 
poralis vagi of van W1jHE’s descriptions. 
In Acipenser, according to VAN WIJHE (No. 36, p. 228), the supra- 
temporal commissure leaves the main infraorbital canal in a lateral 
supratemporal bone, and traverses that bone and then a median supra- 
occipital. The lateral supratemporal is said to have been called the 
occipitale externum by GEGENBAUR and the epiotic by HuxLey. As 
the bone is said to be a purely dermal one, and as the sensory canal 
is said to lie in it, and not superficial to it, it must be the homologue 
of one or both of the lateral supratemporal bones of Polypterus, the 
median supraoccipital of Acipenser being the homologue of the mesial 
supratemporals of opposite sides of the head of Polypterus, fused with 
each other in the middle line. CoLLInGE differs somewhat from 
VAN WIJHE in saying, in one place, that the commissure is given off 
by the main canal as it traverses the “posttemporal” bone of the fish, - 
and, in another, that it leaves that canal near the anterior end of the 
“epiotic” (No. 12, p. 522 and 523). He also says that the suborbital 
