Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/231

Rh pomata (Gill, 1871) have been proposed. Prof. King, con sidering those names to be objectionable, and in some cases inadmissible on certain grounds, in 1873 substituted the name Tretenterata for the first group, the intestine being provided with an anal aperture, while the second group, to which he gives the name of Clistenterata, would embrace animals that are destitute of that organ ; but it must also be remembered that the presence or absence of an anal aperture has been ascertained in only three or four recent genera, and that we are compelled, in a measure, to take for granted that what we find to be the case in Lingula and Tcrebratula is also so in the many extinct families and genera of which the animal cannot be examined. With the character above enounced, we generally find structural modifications of the hinge and other differences in the animal, and especially so in what relates to the muscular system. In the opinion of Prof. King the absence of an anal vent in the Clistenterata makes them inferior to the aniferous Tretenterates.

The animal of the Brachiopod is in all cases protected by a shell composed of two distinct valves ; these valves are always, except in cases of malformation, equal-sided, but not equivalved. The valves are, consequently, essentially symmetrical, which is not the case with the Lamellibran- chiata or Conchifera, so much so, that certain Brachiopod shells received the name Lampadcs, or lamp shells, by 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 shell is likewise most beautiful in its endless shapes and variations. In some species it is thin, semi-transparent, and glassy, in others massive. Generally the shell is from a quarter of au inch to about four inches in size, but in certain species it attains nearly a foot in breadth by something less in length, as is the case with Productus giganteus. The valves are also in some species very unequal in their respective thickness, as may be seen in Productus Llangol- lensis, Davidsonia Verneiulii, &amp;lt;tc., and while the space allotted to the animal is very great in many species, as in Terebratnla sphceroidalis, it is very small in others belonging to StropJiomena, Leptcena, Chonetes, &c. The ventral valve is usually the thickest, and in some forms is six or seven times as great as the opposite one. The outer surface of many of the species presents likewise the most exquisite sculpture, heightened by brilliant shades, or spots of green, red, yellow, and bluish black. Traces of the original colour have also been preserved in some of the fossil forms ; radiating bands of a reddish tint have been often seen in well-preserved examples of Ttrebratida kastata, T. sacttlus, T. communis, T. biplicata, and of several others. Some specimens of T. caruea are of a beautiful pale pink colour when first removed from their matrix, and E. Deslongchamps has described the tint of several Jurassic species. Figs. 12-18. Fin. 12. WulJheimia flavtscent. Interior of ventral valve. /, foramen; rf, dd i- (liiim; t, teeth; a. adductor impressions (= occlusors, Hancock); r, divarieator (= cardinal muscles. King, = muscles diducteurs principal^ Gratiolftl ; c, accessory divaricators (muscles diducteurs acccssoires, (jnitiolet); b, ventral adjustor (= ventral peduncular muscles, or muscles du pedonculc paire superi- i-urc, Gratiolet) ; b, peduncular muscle. FIG. 13. Walilheimitt Jlarescrns. Interior of dorsal valve, r, &amp;lt; , cardinal process, I/, V, liinge-platc; s, dental sockets; /, loop; q, crura; a, a , adductor impres sions; c, accessory divaricator; b, peduncle muscles; is, septum. FIG. 14. Waldhetiniajlate$cent. Longitudinal section of valves. A, ventral, 1!, dorsal valves ; I, loop ; q, crura ; , septum ; c, cardinal process. FIG. 15. Terebratttla vitrea. Interior of dorsal valve. /, loop; 6, hinge-plate; r, cardinal process. FIG. 16. Loop of Tfrefiratttlina caput sfrpentis. FIG. 17. Longitudinal section of Terebratella tlorsata. (References as in fig. 14.) FIG. 18. Longitudinal section ofifagcupwnilvt, The valves have been distinguished by various names, but those of dorsal and ventral are in most general use. The ventral valve is usually the largest, and in many genera, such as Tercbratula and PJiynchonclla, lias 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 deltidiuin. Through the foramen passes a bundle of muscular fibres, termed a peduncle, by which the animal -is in many species attached to submarine objects during at least a portion of its existence. Other forms show no indication of ever having been attached, while some that had been moored by means of a peduncle during the early portion of their existence have become detached at a more advanced stage of life, the opening becoming gradually cicatrized, as is so often seen in Strophomena rhomboidalis, Orthcsina anomala, &amp;lt;tc. Lastly, some species have adhered to submarine objects by a larger or smaller portion of their ventral valve, as is the case with many forms of Crania, Thecidium, Davidsonia, &c. Some Cranias are always attached by the whole surface of their lower or ventral valve, which models itself and fills up all the projections or depressions existing either on the rock, shell, or coral to which it adhered. These irregularities are likewise, at times, reproduced on the upper or dorsal valve. Some species of Strophalosia and Productus seem also to have been moored during life to the sandy or muddy bottoms on which they lived, by the means of tubular spines of greater or lesser length. The interior of the shell varies very much according to families and genera. On the inner surface of both valves several well-defined muscular vascular and ovarian impres sions are observable ; they form either indentations of greater or lesser size and depth, or occur as variously shaped projections. In the Trimerellidce, for example, some of the muscles are attached to a massive or vaulted platform situated in the medio-longitudinal region of (lie 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 modified, thin, calcified, ribbon-shaped lamina or skeleton for the support of the labial or brachial appendages ; and so varied, yet constant in shape to certain species is this laminal apophysis, that it has served as one of the chief characters in the creation 