THE LAMELLIBRANCHIA 215 
and left of the free extremity of the siphonal mass. These “ pallets,” 
as they are called (Fig. 247, IL), probably serve to protect the free 
extremities of the siphons. 
The valves generally bear on their internal surfaces distinct 
impressions of the insertions of the pallial, orbicular, siphonal 
adductor and retractor muscles of the foot, and therefore it is 
possible to infer something of the organisation of the animal in the 
case of fossil species. In general, the anterior side of the shell of 
the Lamellibranchia is the shorter, and the “‘ umbones ” or summits 
of the two valves are directed anteriorly, but in some forms, called 
for this reason “‘ opisthogyrous,” the posterior side is the shorter : 
such are Nucula, Donax, Montacuta, Entovalva, and Cyrtodaria. 
The larval portion of the shell, or “ prodissoconch,” like that of 
the Gastropoda, is often distinct from the following portion, and 
may even be separated from it by a crest 
or ridge (Fig. 196, p): this feature is 
especially well marked in incubatory 
forms with large embryos. Also, in the 
same manner as has been described for 
Gastropods, the external duplicature of 
the mantle border may be reflected over 
the outer surface of, and cover a more 
or less considerable extent of, the shell 
in the Galeommidae and in the endo- 
parasite Entovalva (= Synapticola). By an 
exaggeration of this process the mantle 
may even come to form a closed sac 
around each valve, a feature which may woe 
: om Philobrya sublaevis, right valve, 
be seen in the three genera Hphippodonta, outer aspect. p, prodissoconch. 
Chlamydoconcha, and Scioberetia, and also 
in a species commensal with a Synapta in the Philippines (Semper). 
As in the other classes of the Mollusca, the foot is a muscular 
projection from the ventral surface; its size and form are very 
variable, according to the habit of life adopted by the animal. 
The mass of the foot is frequently invaded by a portion of the 
viscera, at least by a part of the digestive canal, the liver, and the 
gonads, the last-named being superficial. In species in which the 
foot is very mobile, its two lateral faces are united by transverse 
muscular bundles. 
In its most primitive form the foot is a cylinder, more or less 
flattened from side to side, and terminated by a ventral plantar 
surface (Protobranchia, Figs. 230, 231; Pectunculus, Modiolarca, Fig. 
241). But more usually the foot is still further flattened, and 
terminates below in a more or less elongated keel, which may end 
in two points, an anterior and a posterior as in Jvigonia, or in a 
single point, which is always anterior. This latter arrangement is 
Fic. 196, 
