530 ALLIS. (VoL. II. 
the opercular canal is an independent canal not connected with 
the mandibular portion, or with the main line, a condition which 
is permanent in Esox lucius. 
The lateral canals of Amia present a more highly developed 
arrangement than those of the bony fishes. Lepidosteus, judg- 
ing from a most cursory examination, resembles Amia; Polyp- 
terus, except in the possession of large dermal plates, has essen- 
tially the arrangement found in Amia immediately after the 
formation of the primary pores and tubes, and before the opercu- 
lar line has joined the main canal. This larval arrangement in 
Amia also corresponds to that found in the adult of most bony 
fishes, but in many teleostean forms still more primitive condi- 
tions exist. 
Sagemehl has advanced the theory that the teleostean condi- 
tion is derived directly from that found in the adult of Amia by 
the gradual growth of a thicker cutis from the edges of the der- 
mal bones toward their centres. As a result of this growth, the 
bones lose their superficial position, and finally lie beneath a 
thick dermis. The lateral canals, however, in order to maintain 
their communication with the exterior, do not sink in a corre- 
sponding degree, and they are accordingly found in the Teleosts 
much nearer the upper surface of the bone than in Amia. In 
many species they project in ridges above it, and in Gymnotus, 
many of the Murzenidz, some Cyprinoids and others, they lie 
entirely above the bones of the head inclosed in bony tubes 
and forming the ‘“nervenskelet”’ of Stannius. In Polyodon 
Spathula, also, clearly a more primitive form in this respect 
than Amia, a somewhat similar arrangement exists ; for, accord- 
ing to Van Wijhe, the canals lie in the flesh in open, bony 
channels. These different conditions both in the Teleosts and 
in the Ganoids would also be obtained if the development, as 
shown in Amia, was simply arrested instead of undergoing 
retrogression ; that is, by supposing that the Teleosts had never . 
attained the Amia condition instead of having passed through 
it as indicated by Sagemehl. 
In Salvelinus the canal of the lateral line is never developed, 
and the peripheral systems of the cranial canals have, in most 
cases, only a single surface opening corresponding to the pri- 
mary pores of Amia; but in one very large specimen, two of 
these systems had undergone a primary division similar to that 
