440 CEETACEOUS PELECYPODA 



or obliquely truncate. In Spondyhis occasionally thei'e issues from the middle 

 of the terminal disc of the foot a cylindrical tendon, ending in a fleshy mass. 

 In some of the eastern species of this genus the foot is simply thickened a short 

 distance from its base on account of the development of the byssal gland ; the 

 latter has a slight groove, and sometimes a few thick, foliaceous byssal threads 

 are seen to issue from it, but, as a rule, the Spondyli do not appear to spin a byssus. 

 The animal of Pedum, however, always possesses a short byssus, composed of 



thin threads. 



The shells are always inequivalve, the right valve being, as a rule, somewhat 

 larger, more or less attached at the beak, which is often irregularly produced and 

 on the inner side possesses a flattened area ; the left valve is free, generally smaller 

 and also with a smaller hinge area ; both are more or less distinctly eared ; the 

 cartilage is internal, situated in a median pit ; the hinge is toothless or provided 

 with rib-like or tubercular teeth on either side of the cartilage pit, muscular 

 impression subcentral, rounded, more or less confluent with the posterior pedal 

 muscle, which is attached to the inner part of it ; anterior pedal scars sometimes 

 indicated below the beak, but generally not traceable ; pallial line entire ; surface 

 generally ornamented with radiating, often foliaceous ribs and striae ; inner layer 

 of shell thick, more or less glassy. 



In the general habitus the Spondtlid^ more resemble the Pectinid^ than the 

 Padulidm, differing from the former by their more or less sessile habitat, thickness 

 of shell, and by the usual presence of hinge-teeth ; from the latter principally by the 

 inequality of the valves, internal ligament, and by the presence of ocelli at the 

 inner mantle margins. 



In former geological periods the members of this family were not very 

 numerous ; they only appeared Avith the beginning of the mesozoic epoch, but it is 

 difiicult to say whether they attained their maximum of development towards the 

 close of this epoch or not, at least as far as Spondi/lus is concerned, though this was 

 very likely the case, as it certainly was with Plicatiila. Terquemia is only kno^n 

 fossil, and Pedum only recent from a single species. The recent forms are with 

 very few exceptions inhabitants of tropical seas ; they are found mostly in the 

 littoral zone, more rarely at greater depths. 



1. Plicatula, Lam., 1801. Valves radiately plicated, riglit valve more or less 

 largely attached at the beak, which has often a small thickened free area, beaks 

 with indistinct, in advanced age generally quite obsolete, ears, left A-alve somewhat 

 smaller, and usually with a more pointed beak ; hinge with two strong, laterally 

 more or less distinctly striated teeth, interlocking between each other, and a 

 cartilage groove, or pit, between them ; muscular impression single, large, gener- 

 ally well marked, excentric, pallial line entire, distant from the margin ; type, 

 P. plicata, Eorsk. Some of the recent as well as fossil species have barely a trace 

 of attachment at the beak of the right valve. 



la. Deslongchamps, in an elaborate paper on the liassic and Jurassic Plicu' 

 tulce, has pointed out some peculiarities of a certain number of species for which he 



