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-caleareous valves, of which one is dorsal 
Fig, 491, and the other ventral in position. The valves are often thin 
Vertical section of aud of unequal size, but the inequality is never of such a 
B oingua shell, nature as to disturb the bilateral symmetry of the shell. 
showing alternate ‘ : 5 
corneous () and cal- During life the ventral valve, which is commonly the 
careous (0) layers. 3 é é x5 
Strongly magnified larger of the two, occupies an inferior position, and the dorsal 
(ater Gratiolet). a superior. But in describing the shell, it is always so 
oriented that the posterior margin, or /inge-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. 
|) ALL 
es pucuansissan, 
LLL 


The cardinal area is a term applied to the flattened or curved Fic. 492. 
triangular area which is frequently observable between the hinge-line _Cyrtina heteroclyta, 
and the beak (Fig. 492). It is more highly developed in the ventral ae cRn nlenerea 
than in the dorsal valve, and is bisected medially by the triangular ee hae 
delthyrium. 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 deltidivm 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 
