Herrick, Nerves of Siluroid Fishes. 231 
ossicles between the dorsalend of the preoperculum and the 
squamosal. These ossicles are distinct and well formed, but 
in texture are more like a jointed investment of connective 
tissue, for they are slightly or not at all calcified. This same 
remark holds true for some of the specimens of other species 
dissected, though to hardly so great degree. The portion of 
the canal between the squamosal and the post-temporal is 
longer in this species than in any of the othersexamined. The 
investment of the canal here is likewise of very dense, well de- 
fined connective tissue, but not clearly ossified. There is no 
occipital commissure of the canal system. 
It is worthy of remark that the North American siluroid 
fishes comprise a very close group and that the forms here de- 
scribed are all closely related. The species, and even genera, 
are distinguished by very trivial characters and transitional 
forms are very abundant. It is obvious that if slight variations 
in the degree of development of the lateral line canals may 
involve, as in these cases, so important skeletal changes in 
closely related species (and even among individuals of the same 
species) as the development or suppression of several distinct 
cranial bones, then such of the bones of the head as are formed 
in connection with the lateral line canals must be used in 
morphological and phylogenetic speculations with great caution 
and with allowance always made for this surprising variability. 
This series of forms also illustrates in an interesting way how 
elements of the primordial enchondral cranium may secondarily 
assimilate dermal elements. In one specimen of Ameiurus 
melas no lateral canal or ossicles are developed dorsally of the 
preoperculum. In other forms there is a canal in this position 
enclosed in one or more separate dermal ossicles. And finally 
‘in Leptops several of these ossicles have fused with the under- 
lying hyomandibular bone very intimately. | 
Menidia. In the light of these relations I have re-examined 
the bone of Menidia which I have termed the extra-scapular, 
marked ESC on Fig. 5 of my Menidia paper (’99). This bone 
carries the main canal caudad of the squamosal and also the 
short branch which represents the supratemporal or occipital 
