-BRACHIOPODA This class was instituted by M. Cuvier, in consequence Gf an examination of the animal of the Patella unguis of Linnaeus. Its characters are well marked, and abun dantly justify the propriety of the change. The animals of this class are acephalous ; and, like the conchifera, have the cloak divided into two lobes, protected by a bivalve shell. These lobes are free at the anterior mar gin. The branchix consist of small leaves, arranged along the inside of the edge of the cloak, and intimately united with it. From the body, between the lobes of the cloak, issue two arms, fringed with filaments. These are capable of folding up in a spiral form. The mouth is situated between the arms at the base..
All the animals of this class are inhabitants of the sea, and they are permanently attached to rocks and stones. Three genera are known, which are probably the types of as many orders.
I. Lingula. The valves of the shell of the species which constitutes this genus were first figured by Seba, together with the peduncle by which they are support ed. Linnaeus, having seen only one valve, conjectured that it belonged to the Patella, and named it P. unguis. Chemnitz examined both valves, without the peduncle, and pronounced them connected with the genus Pinna. Bugiere, aware of Seba's figure, contemplated the for mation of the new genus for its reception, which La marck executed. M. Cuvier afterwards dissected one of the individuals which Seba had possessed, and un folded characters in its organization, sufficient not only to warrant the construction of a new genus, but a new class.
The peduncle of the Lingula is cartilaginous, having the inferior ends of the two oval valves attached to its extremity, the other end being fixed to foreign bodies. The valves are destitute of teeth, or an elastic ligament, and are opened chiefly by the arms when pushed out, and closed by the adductor muscles, which are capable of acting in an oblique direction, and of giving to the valves a considerable degree of lateral motion. The margin of the cloak, which is double like the shell, is fringed with fine hairs. The arms arc fleshy in their substance, conical, elongated, and compressed in their form, and ornamented on the external surface with thick set fringes or tentacula. The mouth is situated between the arms at their base, and is simple. There is no en largement or the alimentary canal, which can be regard ed as a stomach, and the anus is a simple aperture, situ ated on the side. There are marked indications of sali vary glands and a liver. The blood is conveyed to the gills by two vessels, which are divided at the separation of the lobes into two branches, one of these going to the half of one lobe, and another to the opposite half of the other lobe. Two systemic veins occupy a similar posi
tion, and return the aerated blood to the two lateral sys temic ventricles. The gills themselves are arranged in a pectinated form on the inner surface of each lobe of the cloak. There is nothing known of the nervous or re productive systems of this animal.
The Lingula unguis is the only species of the genus, and appears to be confined to the Indian seas. Some petrifactions have recently been referred to this genus ; but in the absence of all vestige of the peduncle, we do not consider the mere form of the shell as furnishing characters sufficiently obvious and precise to warrant such arrangement.
2. Terebratula. The muscular peduncle of this ge nus passes through a perforation in the largest valve. The arms are shorter than those of lingula, and finked at the extremities. They are supported within by nu merous arcuated plates.
There is one recent species described in the article CONCIIOLOGY) VOL vii. p. 36, genus xxxix. and figured tab. ccvi. fig. 2, as T. vitrea. It appears, however, to be the T. cranium of Muller, figured in Zoologia Da nica, tab. xciv. f. 1. Although we can boast of only one recent species of this genus, our rocks abound in many others which are extinct. Mr. Sowerby, in his valuable work, Mineral Conchology, now publishing, has given excellent representations of several of these. He has been able, from an attentive examination of their form, to construct several new genera, and, from a dissection of the cavity, to unfold the remains of the spiral arms.
3. Crionus. In this genus, first instituted by Poli, one of the valves is membranaceous and flat, and adheres to stones, and the other is flatly conical, and resembles a Patella. The arms nearly resemble those of Lingula. The ovarium, according to Muller, is double and branch ed, and the eggs are round.
The Criopus anomalus is described and figured by Muller under the name Patella anomala, in the Zoologia Danica, tab. v. f. 1-8, and by us, for the first time, as a native of Britain, in the article CONCHOLOGY, vol. vii. p. 65, genus i. sp. 7, P. distorta, tab. ceiv. fig. 4.
It is to be regretted, that so little progress has been made in the examination of the animals of this class. We are still ignorant of their nervous system, and their mode of propagation ; and when these and their other organs shall be more carefully investigated, many new divisions will probably be necessary, as the species in crease in number.