INSECTS. BRACHIOPODA. 
109 
valve is symmetrical in itself, its right ami left halves re- 
sembling one another. On tlie other hand, one valve is nearly 
always larger than the other (Figs. 55 a and 60 a). By the 
larger valve the adult animal is usually attached to rocks or 
other objects. In a few forms, such as Crania (Diagram 1), 
the valve is directly cemented to the rock by its own sub- 
stance. In others, such as Lincjula (Diagram 2 and Fig. 57), 
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 Hemithjris (Diagram 8) and Orhimloidea 
(Fig. 58 h). 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. 59, 60). 
In microscopic structure also the shell differs from tliat 
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 meml)rane. In the 
softer-shelled forms, such as Liwjula, 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 Terehratdla, 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 3n. 
In fossils this structure gives to tlie 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 Atrypidm and most 
Pihynchonellidee. 
The shell-valves are secreted by tlie two mantle-folds 
which line- them. These are extensions of the body-walls, 
and they contain prolongations of the body-cavity, in which 
is a blood-like fluid and in which the generative products 
are formed. These vessels often produce impressions on 
the inside of the shell, and so can he traced in the fossils, 
e.g. Camarophoria (Fig. 56) and a fine specimen of Orthis 
Gallery 
VIII. 
Wall-case 
11c. 
