﻿T. Davidson — What is a Brachioiiod "} 147 



in lengtli, as is the case with Prodtictiis giganteus. The valves are 

 often very unequal in their respective thickness, as may be seen in 

 Prodnctus LlangoUensis (PL VII. Fig. 27) ; Davidsonia Verneuili 

 (PI. VII. Fig*. 26), etc.; and while the space allotted to the animal is 

 veiy great in a number of species, as in Terehratula sphceroidalis 

 (PI. VII. Fig. 25), it is extremely small in many others, as Utropliomena 

 and Leptmia (PL VII. Fig. 28). The outer surface of many of the 

 species presents the most exquisite sculpture, heightened by brilliant 

 shades, stripes, or spots of green, red, j'ellow, 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 manj'' genei'a as Terebratida and BhyncJionella, it 

 has a pi'ominent 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 bundle 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 

 Pi'of. 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 (diftering 

 in this respect from those of the Lamellihrancliiata) , so much so, 

 that certain Brachiopod shells received tlie name of Lampades from 

 some early naturalists ; but while such may bear a kind of resemblance 

 to an antique Etruscan lamp, bj'' 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 bj'- 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 

 Yarj 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 



