THE LAMELLIBRANCHIA 215 



and left of the free extremity of the siphonal mass. These " pallets," 

 as they are called (Fig. 247, II), 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 



T. j.v j.1. n 7 j j Philobrya sublaevis, right valve, 



be seen in the three genera flpfiippoaonta, 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 Trigonia, or in a 

 single point, which is always anterior. This latter arrangement is 



