iS^if^r 



CLASS I PELECYPODA 425 



arched or cycloidally curved. The apex of the cone is formed by the beak 

 or umbo of the valve, the base is the entire margin of the valve. 



The shell of most Pelecypods is composed of several layers of distinct 

 structure. The external layer is usually thin, flexible and dark-coloured, 

 chiefly composed of a horny substance termed conchiolin. This layer is 

 known as the epidermis, or more properly the periostracum ; it is not easily 

 corroded, and hence serves as a protection to the underlying calcareous 

 layers. The outer calcareous layer is composed of prisms of calcite arranged 

 more or less perpendicular to the external surface ; the inner layer is made 

 up of thin, more or less parallel lamellae of porcellanous or pearly texture, 

 disposed at right angles to the general direction of the prismatic layer, 

 and exhibiting the mineralogical characters of aragonite (Fig. 639). Besides 

 the lamellar or prismatic 

 structure, many forms show 

 under the microscope 

 minute, sometimes branched 

 tubulation. 



The variations in shell 

 substance are somewhat 

 characteristic of diff"erent 

 groups. The prisms diff'er p^^^ g39_ 



greatly in size, the Ifirger Veitical section of the shell of Unio. e, b, a, a', the outer pris- 



nr>f>ni'vit.rr in Tnnpm-nmiKi Qnrl matic layer, showing successive increments of shell growth; c, c', 

 ULUUlllug ill lliUU.iailtWb anu the inner lamellar strata. Highly magnitied (after Carpenter). 



Pinna, the smaller in the 



Anatinidae and Myacidae. The prismatic layer is wholly absent in the 

 Chamidae and many other Teleodesmacea ; in the Pectinidae and Limidae 

 the prismatic layer is feebly developed and often recognisable only in young 

 shells. In the Rudistae the prisms run nearly parallel with the outer surface. 

 As aragonite is more soluble than calcite, it frequently happens in fossil 

 shells that the layers composed of the former minei'al have entirely dis- 

 appeared, leaving only the calcitic layers. Pearls are merely loose portions 

 of the inner layer secreted by the mantle surface, usually around foreign 

 bodies which have reached the interior of the shell and set up irritation there. 



In the majority of Pelecypods the valves form a nearly complete defence ; 

 in borers, burrowers and a few degenerate types, the valves cover less and 

 less surface in proportion to that which is bare ; in a few the mantle is re- 

 flected so as to envelop more or less of the outer surface of the valves ; and 

 finally, in Chlamydoconcha, the valves are permanently internal, separately 

 encysted, with the ligament isolated and encysted between them. No 

 example is known of a Pelecypod absolutely destitute of valves in the adult 

 state. 



The valves of the shell are in general substantially equal ; but sometimes 

 they are unequal, especially in sessile or sedentary forms ; and rarely they 

 are spirally twisted, as in Stavelia and Spirodomns. The hinge or articulus 

 comprises the whole articulating apparatus — hinge plate, teeth, ligament, etc. : 

 the primitive hinge, which is coextensive with the ligament, is distinguished 

 by Hyatt as the cardo. The cardinal axis, or right line forming the axis of 

 revolution of the hinge, is parallel with the antero-posterior axis of the 

 animal (as determined by a line drawn through the mouth and posterior 

 adductor) in the ordinary Teleodesmacea ; but in the winged Prionodesmacea, 



