CALLITHEA. Sw. A sub-genus of Mitræ, consisting of those species, which like M. sanguisuga, have the "spire and aperture of nearly equal length; internal channel nearly obsolete; shell with longitudinal linear ribs, crossed by transverse striæ and bands; base contracted." Swainson Mallac. Lard. Cyclop. CALLOSITY. A term used in general zoology to express those hard horny tumidities formed in the skin of some animals, (such as the Dromedary, for instance) in those parts which are most frequently used. It is not used in this sense by Conchologists, who apply it to those undefined tumidities or bumps which appear on the inner surface and hinge of some bivalve shells, and to the thickening over the umbilicus of Naticæ. Glycimeris, fig. 67. Natica, fig. 327, 328. CALPURNUS. Montf. OVULUM _verrucosum_, Auct. Distinguished by the small circular tubercle at the back of each extremity of the shell. Fig. 441. CALYPTRACEA. Lam. A family belonging to the first section of the order Gasteropoda, Lam., the shells of which are described as always external, covering the animal, and having no operculum. The genera contained in this family may be thus distinguished. 1. CALYPTRÆA. Conical; apex central, septum spiral, cup-shaped, or forked; including _Infundibulum_. Fig. 234 to 238. 2. CREPIDULA. Apex terminal; septum flat, reaching half across the aperture. Fig. 239. 3. CAPULUS. Conical; apex obliquely curved, no septum. Fig. 240. 4. EMARGINULA. Apex curved backwards; a notch in the anterior margin; including _Parmophorus_. Fig. 241, 242. 5. CEMORIA. A slit _near_ the apex. Fig. 244. 6. FISSURELLA. A slit _upon_ the apex. Fig. 245. 7. RIMULA. A slit near the margin. Fig. 243. 8. ANCYLUS. Apex curved sidewise. Fig. 246. CALYPTRACEA. Bl. The second family of the order Scutibranchiata. Bl. thus described: "Shell more or less conical, not spiral, or very slightly so; aperture large and entire." The genera included in this family are Crepidula, Calyptræa, Capulus, Hipponyx, and Notrêma. CALYPTRÆA. Lam. _Fam._ Calyptracea, Lam. and Bl.--_Descr._ Conical, patelliform, irregular, with an internal, lateral, salient plate or septum, varying in form.--_Obs._ The internal appendage is in some species cup-shaped, in some it juts out of the centre in a double point; in others it is only a small flap; and in others a spiral disc. These last, which are shaped like Trochus, are separated by De Montfort under the appellation INFUNDIBULUM; TROCHATELLA, Sw. The Calyptræa may be known from Crepidula by the septum, which in the latter is a flat plate reaching half way across the cavity. Fig. 234, 5, 6. CAMERINA. Brug. NUMMULITES, Auct.