476 THE PALEONTOLOGY OF MINNESOTA. 
(Lamellibranchiata. 
of the foot, and another in the back serving for beth the inhalation of water and the 
expulsion of the excrements. The posterior opening may be further modified so as 
to form two more or less distinct tubes or siphons, and these may be retractible or 
of such size and consistency that they project permanently through the gaping 
posterior margin of the shell. In most instances the siphons are capable of being 
completely or partially retracted, and the line of attachment of the muscles of the 
mantle producing this retraction is bent inward more or less decidedly. When such 
an inbending of the pallial line (as the attachment of the mantle to the inner surface 
of the shell is called) is found in fossil shells the inference is regarded as conclusive 
that the animal possessed retractile siphons. When, on the other hand, the pallial 
line is simple (7. e., without a sinus) we are obliged to conclude that the siphons were 
either very small or wanting entirely. 
The foot—a perfectly retractile organ, presumably of locomotion—lies in the 
anterior part of the shell between the gills and mantle lobes. Its form is various, 
but commonly compressed, hatchet or club-shaped, and the muscles which produce 
and regulate its action are attached usually above or behind the anterior adductor. 
Not infrequently chitinous threads spring from the lower side of the foot. When 
these are developed in sufficient number to form a bundle or byssus, the shells may 
thereby attach themselves to foreign bodies, and in such cases the anterior margins 
of the valves do not close tightly, but leave what is known as the byssal opening. 
Among paleozoic representatives of this class the Ambonychiide afford the best 
instances of shells with a byssal opening. 
Of all the organs of the animal none are of greater importance to the paleon- 
tologist than the strong muscles (adductors) which serve to close the valves. There 
may be only one, the posterior, as in the recent oyster, or of the two the anterior 
one may be disproportionately small. In the majority of cases, however, the two 
muscles are approximately of equal size. Other and much smaller muscular scars 
may be noticed, especially in the umbonal cavity. which were produced by muscles 
which partially supported the movements of the gills and palpi and, as already stated, 
of the foot. 
The shell in which the interest of the paleontologist is chiefly centered consists 
largely of two layers, the outer, secreted by the thickened margin of the mantle, 
being composed of vertically arranged prismatic cells filled with calcite, the inner of 
structureless thin parallel leaves. Generally a delicate chitinous epidermis is spread 
over the cellulose layer. Growth of the valves begins at the apex or beak, a more 
or less prominent point situated almost invariably somewhere along the anterior 
half of the hinge margin. Further increase takes place principally at the periphery, 
producing, when the edges of the mantle are entire, a simple, more or less regularly 
