1899. 1 Foord. — Carboniferous Brachiopoda and Molhisca. 73 



their prey, consisting of fishes and Crustacea whose shells they 

 crush with their powerful, parrot-like beaks. The foot, I 

 have already referred to. Another important organ is the 

 mantle, or pallium, which secretes the calcareous .shell, when 

 present, and encloses the viscera ; the mark left by this organ, 

 hence called the pallial-line, seen on the inside of many 

 L,amellibranch shells, is of importance, as its outline when 

 stretching without interruption from one muscular impression 

 to another indicates that the animal inhabiting the shell was 

 not possessed of respiratory siphons, while if it is interrupted 

 by a sinus or indentation, these siphons were present. As 

 the pallial line is often preserved in fossil shells, it is thus of 

 some assistance in classification. The shell, almost the only 

 part of these animals preserved in the rocks, varies, as might 

 be expected, greatly in shape. Jn the I,amellibranchs it 

 consists of two pieces or valves, round or oval in shape, and 

 held together by the elastic " ligament " which may be internal 

 or external. Where the semicircular or concentric lines on 

 the surface are smallest there is a more or less prominent 

 hump ; this is called the beak or umbo, and represents the 

 oldest part of the shell ; beneath this, inside the shell, 

 there is a shelly plate bearing prominences called teeth with 

 corresponding depressions into which they fit in the opposite 

 valve ; the whole is called the dental plate or hinge. When 

 closed the valves completely enwrap the animal ; in those 

 forms, however, which are siphonate, the valves often " gape" 

 at one end, the posterior, or that farthest from the umbones. 

 This is especially the case with the burrowing tribes, in which 

 the siphons are often extended far beyond the shell. The 

 substance of the shell itself is made up of three layers, the 

 internal, laminated layer, called the nacreous or mother-of- 

 pearl layer ; the middle or prismatic layer, and the outer or 

 horny layer — periostracum — which protects the shell from the 

 dissolving action of water. The first is secreted by the 

 surface of the mantle, the other two by its free edge, which 

 produces all the various kinds of ornamentation, ribs, nodules, 

 striations, spines, &c, upon the surface of the shell, to effect 

 which its outline undergoes changes of a like protean 

 character. 



The shell in the Scaphopods is shaped like an elephant's 



