Brachopoda. 63 



in itself, its right and left halves resembling one another. On CrALtEEY 

 the other hand, one valve is nearly always larger than the other j.gjj|. gj^^ 

 (Fig. 95«). By the larger valve the adult animal is usually wall-casea 

 attached to rocks or other objects. In a few forms, such as 10 & 11, 

 Crania (Diagram 1), the valve is directly cemented to the rock 



by its own substance. In others, such as Lingula (Diagram 2 86-b8. 

 and Fig. 100), the valve is attached by a long muscular stalk, the 

 "peduncle" or "pedicle," which is capable of waving movement 

 and of contraction. There are also intermediate stages, with 

 shorter peduncles, such as Semithyris (Diagram 8) and Bucina 

 (Fig. 97). This larger, attached valve is therefore often called the 

 "peduncle valve": by English writers it is called the "ventral 

 valve," although in the natural position of stalked forms it is 

 always the uppermost. Similarly the smaller valve is called the 

 "dorsal valve"; but a better name is "brachial valve," which 

 reminds one that this valve often bears a calcareous skeleton 

 supporting the so-called " arms " (Diagram 9 and Figs. 101 and 102). 



In microscopic structure also the shell differs from that of the 

 Mollusca (Diagram 3). It is mainly composed of small prisms 

 of calcite (carbonate of lime), which usually lie at right angles 

 to the surface of the shell. In the harder-shelled forms these 

 make up the greater part of the shell, merely being coated on the 

 surface with a layer of slightly different texture and with a thin 

 horny membrane, the " periostracum. " In the softer- shelled 

 forms, such as Lingula, horny substance occurs not merely on the 

 surface, but in layers between the prisms, which in this case are of 

 phosphate of lime. In many genera, such as Terebratella, Crania, 

 Cistella, the shell is perforated by a number of small canals ; these 

 contain processes of the mantle, the arrangement of which is shown 

 in Diagram 2>a. In fossils this structure gives to the exterior of the 

 shell a pitted or " punctate " appearance under a magnifying glass, 

 and thus enables one to distinguish such shells from those which are 

 "impunctate," as the shells of Atrypidae and most Khynchonellidae. 



In those Brachiopoda that appear to be the simplest and oldest, 

 the shells are not as a rule joined by any hinge (Diagram 7). 

 These have therefore been called Inaeticulata or Ecaedines * : 

 they include Lingula, Bisana, Oholus, Crania, Trimerella, and their 

 allies. In more advanced forms, such as Orthis, Leptcsna, Atrypa, 



^ £, without ; cardo, a hinge. 



