294 MOLLUSCOIDEA SUB-KINGDOM v 



or absence of tubuli, Brachiopods are distinguished as punctate or im- 

 punctate. 



The Craniidae have thick shells composed of concentric layers of carbonate 

 of lime. In Crania the shell substance is homogeneous, and punctured by 

 distally branching canals. In the Lingulidae and Obolidae the shell consists of 

 alternating layers of phosphate of lime, admixed with lime carbonate, and a 

 lustrous horny substance known as ceratin. The calcareous layers are pris- 

 matic, and are traversed by fine tubuli (Fig. 491). It is believed that the 

 function of the punctae is for respiration, but the fact that 

 these extensions of the mantle are not exposed to the water 

 may not accord with this explanation. 



Valves. Brachiopods are delicately constituted animals, 

 covered by two very vascular mantle lobes which secrete the 

 calcareous or corneo-calcareous valves, of which one is dorsal 

 FIG. 491. an d the other ventral in position. The valves are often thin' 

 vertical section of and of unequal size, but the inequality is never of such a 

 showing 5 "'' 'alternate nature as to disturb the bilateral symmetry of the shell, 

 corneous () and cai- During life the ventral valve, which is commonly the 



careous (b) layers. , J , 



strongly magnified larger of the two, occupies an inferior position, and the dorsal 

 let) ' a superior. But in describing the shell, it is always so 

 oriented that the posterior margin, or hinge-line, is placed above, and the 

 anterior one below. A line drawn from the beak to the anterior margin 

 describes the length; and one at right angles to the same, in the direction of 

 right and left, the width; a third line drawn perpendicularly to the other 

 two, and passing through the centres of the valves, measures the thickness. 

 In the Protremata and Telotremata the ventral valve is convex, and curved in 

 such manner at the posterior margin as to form a beak. The beak may 

 be pointed, or it may be perforated by a round opening, or foramen, for 

 the protrusion of the pedicle. In many cases, however, the pedicle opening 

 lies underneath the apex of the beak, and sometimes encroaches upon a 

 portion of the dorsal valve. In the Atremata the pedicle emerges from 

 between the two valves ; in the Neotremata the posterior margin of the 

 ventral valve is notched, or there may be a small, circular, eccentric per- 

 foration, or a more or less long, narrow slit. In the Telotremata 

 the pedicle opening, or delthyrium, which is originally triangular 

 in form, becomes either wholly or partially closed by the 

 growth of deltidial plates. In the Protremata and certain of the 

 Neotremata, the delthyrium is closed by a true deltidium, as 

 this covering is called. 



The cardinal area is a term applied to the flattened or curved FIG. 492. 

 triangular area which is frequently observable between the hinge-line Cyrtinahetewdyto, 

 and the beak (Fig. 492). It is more highly developed in the ventral ^^^hteharea 

 than in the dorsal valve, and is bisected medially by the triangular aad 5**ttdodel- 

 deltliyrium. A true cardinal area is absent in the Atremata and 

 Neotremata; but when a small area is present in these orders, it is called a false 

 cardinal area (Fig. 506, B, C). A split tubular structure, or syrinx, which partially 

 enclosed the pedicle, is developed in the delthyrium of some spire-bearing forms. 



The deltidium has its origin in the Cephalula stage (Thecidea Mediterranea) 

 contemporaneously with the rudiments of the dorsal and ventral valves, while the 

 embryo is still in the free, swimming condition. The dorsal valve and incipient 



