T. Davidson — What is a Brachiopod ? 
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in length, as is the case with Productus giganteus. The valves are 
often very unequal in their respective thickness, as may be seen in 
Productus Llangollensis (PI. VII. Fig. 27) ; Davidsonia Verneuili 
(PI. VII. Fig. 26), etc.: and while the space allotted to the animal is 
very great in a number of species, as in Terebratula sphceroidalis 
(PI. VII. Fig. 25), it is extremely small in many others, as Ütropliomena 
and Leptccna (PI. VII. Fig. 28). The outer surface of many of the 
species presents the most exquisite sculpture, heightened by brilliant 
sliades, stripes, or spots of green, red, yellow, and bluish-black. 
The valves have been distinguished by various names, but those 
of dorsal and ventral are in more general use. The ventral valve is 
usually the larger, in many genera as Terebratula and Bhynchonella, it 
has a prominent beak, with a circular or otherwise shaped Perforation 
or foramen at or near its extremity, partly completed by one or two 
plates termed a deltidium. Through the foramen passes a bündle of 
muscular fibres called the peduncle, by which the animal is in many 
species attached to sub-marine objects during at least a portion of its 
existence. It is, however, certain from the admirable researches of 
Prof. Morse that the embryo of some, if not of every species, 
swims most actively in every direction and turns abruptly about ; 
but that in the fourth stage of its development it becomes attached, 
the peduncle widening at its end into a sucking disk. Other species 
show no indication of ever having been attached ; while some that 
had been fixed by means of a peduncle during a part of their exist- 
ence become detached at a more advanced stage of life, the opening 
for the peduncle becoming gradually cicatrized. Lastly, forms 
appear to have adhered to sub-marine objects by a larger or smaller 
portion of the surface of their ventral valve, or by spines ( Stro - 
phalosia, etc.), during their entire life. 
In external shape the valves are essentially symmetrical (differing 
in this respect from those of the Lamellibranchiata), so much so, 
that certain Brachiopod sliells received the name of Lampades from 
some early naturalists ; but while such may bear a kind of resemblance 
to an antique Etruscan lamp, by far the larger number in no way 
resemble one. The valves are either articulated by means of two 
curved teeth developed from the hinge-margin of the larger valve, 
and fitting into sockets in the corresponding part of the smaller one, 
or they are unarticulated, and kept in place by means of muscles 
especially provided for that purpose. 
Having described the exterior of the shell, a few words may now 
be devoted to its interior. On the inner surface of both valves 
several well-defined muscular impressions are observable, which 
vary considerably in position and shape in different genera and 
species. They form either indentations of greater or lesser size and 
depth, or occur as variously shaped projections. In the TrimereUidce 
some of the muscles are attached to a massive or vaulted platform 
situated in the medio-longitudinal region of the posterior half or 
umbonal portion of both valves. In addition to these, there exists 
in the interior of the dorsal valve of some genera a variously modi- 
fied thin, calcified ribbon-shaped lamina or skeleton for the support 
