514 WALTER EDWARD COLLINGE. 
head, rostrum, and gill-flaps in the greatest profusion (Pl. 39, 
fig. 5). 
On the specimen in which the cluster-pores were counted 
there were about 3500 groups of primitive pores, the number 
of pores in each group varying from seven to seventeen. 
The diameter of a pore was almost equal to the distance 
from the surface to the sensory organ lying at its base. 
Histologically they appeared to be miniature cluster-pores. 
It has been very generally supposed that the sensory organs of 
the lateral canal differed from those in the canals of the head. 
Excepting in size I have been unable to distinguish any dif- 
ferences worthy of note in those in Polyodon and Acipenser. 
Ewart (26) states that this difference does not exist, and in 
Raia finds that “ parts of the cranial canals exactly agree in 
structure with the canals of the trunk” (p. 98). 
So far as I have been able to ascertain, these primitive pores 
are found only in the Selacheoid Ganoids in the form I have 
figured on Pl. 39, figs. 2 and 3, p.p., viz. in series aggregated 
into distinct pigmented patches. 
vil. INNERVATION OF THE CANALS. 
The cranial nervous system of Polyodon exhibits a number 
of interesting features at present not known to occur in any 
other Ganoid. 
From the large number of sensory organs I expected to find 
great branching of the facial nerve and possibly the trigeminal 
also, and generally a condition not unlike that figured by Ewart 
in Lemargus (25). Indeed, I have endeavoured to inter- 
pret the cranial nerves in the light of this author’s recent 
researches, which have placed the study of the cranial 
neurology of fishes and Vertebrates generally in quite a new 
aspect. 
In Polyodon there are three features which have greatly 
modified the number and distribution of the cranial nerves, viz.: 
(i) The unusual number of sensory organs ; . 
(ii) The backward position of the suspensorium (hyomandi- 
bular) ; and 
