OVULUM. Brug. (_Ovum_, an egg, dim.) _Fam._ Convoluta, Lam. Angyostomata, Bl.--_Descr._ Ovate or fusiform, smooth, convolute, spire covered; aperture narrow, with a canal at each extremity; outer lip crenulated, inflected; inner lip smooth, callous towards the spiral extremity; dorsal area wide, sometimes indistinctly marked.--_Obs._ The Ovula were placed by Linnæus in his genus Bulla, from which they are very remote. They differ from Cypræa in having the inner lip smooth. We have given representations of their different forms as follows: O. Ovum, fig. 442. O. verrucosum, (Calpurnus Montf.) fig. 441. O. Volva, the weaver's shuttle (Radius, Montf.) fig. 442. O. gibbosum, (Ultimus, Montf.) fig. 443. OXYSTOMATA. Bl. The fifth family of Asiphonibranchiata, Bl. This family appears to have been formed for the express purpose of providing a place in the system for the genus Janthina, which seems to bear so little analogy with other genera of Mollusca, that conchological writers have been puzzled to know where to place it. PACHYLABRA. Sw. PACHYSTOMA, Guild. A sub-genus of Ampullaria, the outer lip of which is thickened within. _Ex._ Ampullaria globosa. PACHYMYA. Sow. ([Greek: pachus], _pachus_, thick, and _Mya_.) _Fam._ Cardiacea? Lam.--_Descr._ Obliquely elongated, equivalve, thick, sub-bilobed, with beaks near the anterior extremity; ligament partly immersed attached to prominent fulcra.--_Obs._ This singular fossil is shaped like Modiola, but the shell being extremely thick, and the ligament attached to a prominent fulcrum, it is difficult to know where to place it. Fig. 130, Pachymya Gigas. PACHYSTOMA. Guild. ([Greek: pachus], _pachus_, thick; [Greek: stoma], _stoma_, mouth.) A genus composed of such species of Ampullaria, Auct. as have the edge of the aperture thickened and grooved within so as to form a sort of ledge upon which the operculum rests. Ampullaria globosa and corrugata are examples of this variation. The name Pachylabra is given to such species by Swainson, who objects to the above name on account of its having been previously used to a genus of fishes. Fig. 539. PACHYTOMA. Sw. A sub-genus of Helicina, thus described, "Aperture entire; the inner lip very thick; the spiral whorls hardly convex; P. occidentalis. Zool. J. iii. 15. f. 6-10. viridis, Zool. Journ. i. pl. 6. f. 7." Sw. p. 337. PACLITES. Montf. A genus composed of species of Belemnites, Auct. described towards the extremity, with a pore, at the apex, and a straight lengthened aperture. _Ex._ B. ungulatus, Bl. PADOLLUS. Montf. A genus composed of species of HALIOTIS, with a strongly marked spiral groove. _Ex._ H. tricostalis, Lam. Fig. 339.