278 THE LAMELLIBRANCHIA 
which extends from the anterior adductor muscle to the point 
of separation of the two siphons, and surrounds and is continuous 
with the foot. This septum, therefore, has exactly the situation 
and the relations of the branchial septum of the majority of the 
Lamellibranchia, which divides the pallial cavity into two chambers. 
The group is derivable, more or less directly, from the Anatinacea, 
through the series Lyonsia, Lyonsiella, Poromya, Cetoconcha, Cuspidaria, 
in which one may observe a gradual increase in the amount of 
muscular fibre in the gill filaments or in their reduced equivalents, 
so that any objection to the branchial origin of the septum, because 
of its muscularity, cannot hold good. The muscular septum is 
inserted on the shell, especially in the neighbourhood of the two 
adductor muscles. The origin of the anterior and posterior exten- 
sions of the septum and of its muscular attachments to the two 
extremities of the shell is to be found in the physiological contrac- 
tions necessary to create a current of water on the respiratory 
surface of the supra-septal chamber. The septum is, in fact, always 
pierced by paired orifices, which admit of the passage of water. 
The Septibranchia are all marine, 
inhabit considerable depths of the 
sea, and are carnivorous. The order 
only comprises one sub-order, the 
Poromyacea. 
FamMIty 1. Poromyipasz, Dall. 
Siphons short and separate ; the bran- 
chial siphon provided with a large 
valve. Foot pointed and not byssi- 
ferous. The branchial septum bears two 
groups of transversely elongate orifices 
on either side; these are formed by 
a few branchial filaments, with or with- 
out junctions. The palps are large. 
All the members of the family are her- 
maphrodite. Genera—Poromya, Forbes ; 
no pallial sinus; British (Fig. 249). 
Fic, 250. Dermatomya, Dall; a_ pallial sinus 
_Aventral view of Cetoconcha, removed present.  Liopistha, Meek ; fossil from 
from its shell. «a.o, anterior septal 
orifices ; a.p, anterior palp ; f, foot ; Mm, the Cretaceous. Faminy 2. CErrocon- 
mouth m-osmeitian septal ontces: 2, oHrpax, Ridewood, Branchial septum 
p.P, posterior palp ; Se, branchial sep- bearing three groups of orifices on each 
(nee Maeuny? branchial siphon. side ; these orifices are separated by 
rudimentary branchial filaments. Palps 
large ; siphons short, separate, the branchial siphon with a valve. Genus 
—Cetoconcha, Dall (= Silenia, Smith) ; abyssal (Fig. 250). Faminy 3. 
CUSPIDARIIDAE, Fischer. Siphons long and united, their extremities 
surrounded by tentacles. Foot narrow, with a rudimentary byssus. 
Palps greatly reduced or absent. Branchial septum pierced by four or 
