524 WALTER EDWARD COLLINGE. 
around the anterior border of the rostrum, joining with its 
fellow of the opposite side. 
These bones which I have spoken of as drain-pipe-like canal 
bones are exceedingly interesting in that they probably repre- 
sent the earliest trace we have of an ossified investment of the 
sensory canal system. 
In Polyodon and Psepherus they are long thin-walled 
bony channels in the greater portion of the head, none occur- 
ring on the lateral canal. They never assume the disc-like 
form found in Acipenser (Pl. 39, fig. 7, c). They average 
from 3 to 9 mm. in length, some few on the dorsal surface of 
the head being much larger. 
In Acipenser they are present both on the head and in 
connection with the lateral canal. Those on the head vary in 
shape and size, some being pipe-like in form, others irregular. 
The simplest form in which they were found is shown in 
fig. 7, c (Pl. 39). These were taken from the anterior portion 
of the sub-orbital branch. They are thin disc-like pieces of 
bone measuring from 43 to 7 mm. across and about + mm. in 
thickness. Similar forms, only thicker, were found in the same 
species conducting the lateral canal. 
8. The Pores.—In Acipenser there are no structures 
that can be regarded as cluster-pores. On the ventral surface 
of the rostrum there are numerous patches or groups of primi- 
tive pores which are similar in all respects to those previously 
described in Polyodon. In a sturgeon measuring 10 feet 
3 inches in length upwards of eight hundred of these groups 
were counted. 
xi. SUMMARY AND CONCLUSION. 
In summarising the more important features resulting from 
this investigation, it will already have been observed that 
perhaps the most important evidence obtained is that bearing 
upon the Elasmobranch character of Polyodon and of the 
Selachoid Ganoids generally. 
The Selachoid Ganoids are the first group of fishes as yet 
