HeErRRIcK, Werves of Silurotd Fishes. 225 
and, as we shall see in the next sub-section, it is present in 
some, but not all, of our-other common North American silu- 
roid fishes. In A. nebulosus it is present in some individuals, 
but absent in others. The only bone in this region belonging 
to the lateral line system which I find described in the literature 
is the extra-scapular, or supra-tempora]. But thisbone, or chain 
of bones, as the case may be, as has been clearly brought out 
by ALLIs (’99), is characterized by the presence within it of the 
supra-temporal cross-commissure of the lateral line canal sys- 
tem, and this commissure is totally wanting in all of the North 
American siluroids which I have examined, as appears beyond. 
This of course makes it difficult to compare this ossicle with the 
extra-scapular, and yet the difficulty is not, perhaps, an insuper- 
able one. For in Amia, where there is a single extra-scapular 
bone, the main canal traverses its base and contains within it a 
sense organ (the 18th. infra-orbital of ALLis’ nomenclature), 
while the supra-temporal commissure occupies the transverse 
limb of the same bone. In Gadus (CoLE, ’98), where there are 
several bones in the extra-scapular series, the main canal trav- 
erses the outer one, containing within it a sense organ, and the 
same is true in Polypterus (ALI1s, ’00) and in Salmo salar 
(BrucH, '75). It appears to be probable, therefore, that the 
ossicle in question corresponds to the lateral extra-scapular or 
supra-temporal bone of these fishes, the other bones of the 
series having disappeared with the loss of the commissural 
canal. 
Behind the post-temporal bone there are two (three in one 
of my specimens) essentially similar tubular bones (Fig. 1, 
LOS. 1 and LOS. 2), as mentioned above, each carrying 
one organ of the lateral line. They are long and slender, 
scarcely larger than the membranous canal which they enclose, 
and lying free in the sub-dermal tissue. Caudad of this point 
the canal lies embedded in the dermis at the level of the inter- 
muscular septum between the dorsal and lateral musculature. 
Its lumen is much less than that of the cranial canals. The 
sense organs are arranged in the canal quite regularly through- 
out the trunk and between each two organs the canal communi- 
