THE SENSORY CANAL SYSTEM OF FISHES. 527 
lateral canal in Polyodon and Psepherus, developed to 
a greater extent in such forms as Tenura lymna, Dasy- 
batus dipterurus, Alopias vulpes, Rhinobates plani- 
ceps, &c.! | 
The persistence of dermal canals in Polyodon and 
Psepherus, and the very slight development in these two 
forms, and complete absence in Acipenser, of any true man- 
dibular or maxillary branches, are prominent characteristics of 
the Selachians and Batoidea. In the distribution and form of 
the sensory organs (cluster pores, &c.) there are many points 
of agreement. 
The total absence or only slight development in the Elasmo- 
branchs of commissures agrees with the condition found in 
Polyodon and Psepherus, whilst in Acipenser the pre- 
sence of only one—the occipital—and the arrangement of 
the canals and branches in the commissural regions of all 
three genera, exhibits many close affinities to such forms as 
Heptabranchias maculatus, &c. 
The number, form, and distribution of the cranial nerves of 
Poly odon resemble, in the branching of the buccalis division 
of the facial nerve, and in the distribution of the otic, &c., 
the condition described and figured by Ewart (25 and 26) in 
the Elasmobranchii. 
Indeed, after the many anatomical relations that Polyodon 
exhibits with the Elasmobranchii—described by Bridge some 
years ago (16)—I should have been greatly surprised had 
there not been a similar agreement in the sensory canal 
system. 
xiv. CoMPARISON AND REVIEW OF THE SYSTEM IN THE 
GANOIDEI. 
In comparing the sensory canal systerm in the existing 
Ganoids, it must be borne in mind that they are an order of 
fishes the families of which are widely separated from one 
1 The sensory canal system of all these forms has been carefully figured by 
Mr, 8. Garman (82). 
