526 WALTER EDWARD COLLINGE. 
number of features common to both. These may be summarised 
as follows: 
Polyodon differs from Acipenser in— 
(a) The canals being largely dermal ones, e.g. the lateral 
canal part of the main canal of the head and its branches—none 
passing through any of the cranial elements. 
(6) The presence of branches on the lateral canal and on the 
hyomandibular branch of the main canal. 
(c) The absence of commissures. 
(d) The peculiar course and coalescence of the supra- and 
sub-orbital branches, by which a single canal is formed which 
does not pass along the lateral borders of the rostrum, but on 
either side of the parasphenoid. 
(e) The presence of small mandibular and maxillary branches, 
a feature of great interest considering the many Elasmobranch 
affinities of the fish. 
(f) The presence of cluster pores and other sensory organs 
on the lateral canal. 
The sensory canal system of Polyodon agrees with that of 
Acipenser in— 
(a) The presence of a lateral and main canal with supra- and 
sub-orbital branches. 
(b) The presence of a series of small bony elements—drain- 
pipe-like canal bones—conducting either the canals or their 
branches. 
(c) The presence of primitive pores upon the head. 
(d) The absence of either a pre-orbital or an ethmoidal com- 
missure. 
xiii. CoMPARISON OF THE SELACHOID GANOIDS WITH THE 
ELASMOBRANCHII. 
A comparison of the sensory canal system of the Selachoid 
Ganoids with that of the Elasmobranchii shows that there 
are many important features common to the two. 
The distribution and number of the canals and branches 
in all the Selachoid Ganoids are undoubtedly of an Elasmo- 
branch character. Firstly, there is the branching of the 
