CRICOSTOMATA. 131 



more free from the sides of the shell, so that, instead of forming 

 a regtdar plate, covering half the aperture, it assumes a variety 

 of shapes, and in some is cup -shaped, in others forked, and in 

 some forms a little angular shelf. Indeed, the varial ions are so 

 numerous that I think it would be better to throw the two 

 genera into one, and then divide them into smaller groups. 

 Some species of Calyptraea are farther removed from each other 

 with respect to the characters of the septum and general form 

 of the shell, than they are from the Crepidulse. Fig. 239. 

 Mediterranean, North and South America, East and "West 

 Indies, JSTew South Wales, &c, PI. xii. fig. 239. 



CEEPIDULINA. Bl. Cristellaeia, Lam. Microscopic. 



CEESEIS. Eang. 1828. Order, Pteropoda, Lam.— Descr. Thin, 

 fragile, transparent, pyramidal, pointed ; with a dorsal ridge 

 produced into a poiat at the edge of the apertxu-e. — Ohs. The 

 species found ia the Mediterranean is named C. Spinifera, from 

 its resemblance to a thorn. PL xii. fig. 222. 



CEEUSIA. Leach. (Creux, se. Er. a cavity.) Fam. Balanidea. 

 Bl. Order, Sessile Cirripedes, Lam. — Descr, A depressed cone, 

 consisting of four valves, supported upon, and jointed to, a cup- 

 shaped cavity formed in the Madrepores, in which it resides. 

 Aperture quadrilateral, closed by an operculum of four valves. 

 — 06s. This genus is distinguished from Pyrgoma, which is sup- 

 ported on the edge of a similar cup-shaped cavity, by the paries 

 being composed of four valves, whereas in Pyrgoma, it consists 

 of a single piece. East Indies. C. G-regaria. PL i. fig. 28. 



CEICOSTOMATA, BL The second famHy of Asiphonibran- 

 chiata, Bl. It is thus described : " shell equally (with the 

 animal) variable in general form, but of which the aperture, 

 always nearly round, is completely closed by the shelly or horny 

 operculum ; whorls few, and apex sublateral." This family 

 agrees in some measure with the family Tui'binacea of Lamarck, 

 and with the genus Turbo in the system of Linnaeus. It con- 

 tains the genera Pleurotomaria, Delphinula, Turritella, Proto, 

 Scalaria, Yermetus^ Siliquaria, Magilus, Valvata, Cyelostoma, 

 and Paludina. 



K 2 



