BRACHIOPODA. 287 



and much longer appendage, usually spirally convoluted, and edged, like 

 the arms, with a fine and'close fringe. The mouth is a small vertical fiss- 

 ure between these tliree large appendages. The principal part of the 

 body, situated near the hinge, contains the numerous muscles which reach 

 from one valve to tlie other, and between them are the viscera, which oc- 

 cupy but little space. 



Numberless Terebratulae are found fossil or petrified, in certain secondary 

 strata of ancient formations. The living species are less numerous. 



The shell of some is transversely broader or longer, in a direction per- 

 pendicular to the hinge, with an entire or emarginated contour, with two 

 or several lobes; some of them are even triangular; the surface is smooth, 

 sulcated in radii, or veined; they are thick or thin, and even diaphanous. 

 In several of them, in lieu of the hole in the summit of the thin valve, there 

 is a notch, and this notch is sometimes partly formed by two accessory pie- 

 ces, &c. It is probable that when better known their animals will present 

 generic differences. Already in the 



Spibifeh, Sowerby, 

 Two large cones have been recognized, formed of a spiral thread, which 

 appear to have supported the animal. In 



Thecidea, Def. 

 The pedicle seems to have been incorporated with the small valve. 



Okbicula, Cuv. 

 he Orbiculsc have two unequal valves, one of which, that is round and coni- 

 cal when viewed by itself, resembles the shell of a PateUa; the other is flat 

 and fixed to a rock. The arms of the animal, — Criopus, Poli, — are ciliated 

 and spiraUy recurved hke that of the Lingulae. 



Crania, Brug. 



Should be approximated to the Orbiculee. The arms of the animal are 

 also ciliated, but the shells have deep and round internal muscular impress- 

 ions, that have caused it to be compared to the figure of a skull. 



One of them inhabits European seas; Anomia craniolaris, L. Several are 

 fossil; such as the Cran, antiqua, and the others of which M. Hceninghaus 

 has given an excellent Monograph. 



