PRODUCTIVE ; CRANIID^E ; DISCINID^. 423 



the canal, is forced onwards up the tube, and thus causes the 

 arm to project. We shall hereafter meet with a similar con- 

 trivance, in the tubular feet of the Echinodermata ( 1097). 

 The Terebratulida? are found in all seas, from the polar to the 

 equinoctial, at a depth of from ten to ninety fathoms, or even more. 



105 1. The PRODUCTIVE, which are only known in a fossil 

 state, appear to have been generally unattached, and their valves 

 .are united by a long hinge-line, which however is destitute of 

 teeth. They are remarkable for the form of their shells, the 

 lower valve being convex, and the upper concave externally ; so 

 that the entire shell forms a concavo-convex figure, and the inner 

 surfaces of the two valves are brought very close together. 

 They are often armed with spines along the hinge-margin. The 

 CIIANIID^E, of which several species are still in existence, have 

 neither a hinge nor a peduncle, but the shells are attached by 

 the lower valve. The dorsal valve is the largest, convex, and 

 limpet-like ; and the interior of both valves exhibits a broad 

 granulated border, and four large muscular impressions. In the 

 DISCINID^E, which are nearly allied to the preceding, the two 

 valves also differ considerably in form and size ; the upper one 

 being conical and rounded, like the shell of the Limpet (with 

 which it was formerly confounded), whilst the other is flat, with 

 a fissure near the centre for the passage of a ligament, by means 

 of which it is fixed to the rocks. The Mollusk has ciliated 

 arms, rolled up spirally when withdrawn, and the mantle is 

 fringed all round with long horny bristles. The recent species 

 of this genus are found attached to stones, shells, and sunken 

 wrecks, at various depths, down to seventeen fathoms. 



1052. In the Family LINGULIDJE, the animal is attached by a 

 peduncle, which is often of considerable length, but which issues 

 from between the valves of the shell at the hinge-margin, and 

 not through an aperture. The valves are of a more or less horny 

 texture, nearly equal, and destitute of hinge-teeth ; and the 

 mantle is fringed with horny bristles. The recent species all 

 belong to the genus Lingula, so called from the resemblance in 

 the shape of their shells to a tongue ; they are found in shallow 

 water, on the shores of the Indian and Pacific Oceans. 



