PAGODELLA. 213 



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 Am- 

 pullaria, the outer lip of which is thickened within. Ex. Ampul- 

 laria globosa. 



PACHYMYA. Sow. (Traxve, pachus, thick, and Mya.) Fmn. 

 Cardiacea ? Lam. — Descr. Obliquely elongated, equivalve, thick, 

 sub-bilobed, with beaks near the anterior extremity ; ligament 

 partly immersed attached to prominent fulcra. — Obs. This singu- 

 lar 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, (jra-xyG, packus, thick ; oro/za, 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. Am- 

 pullaria 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 pre- 

 viously 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. pi. 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. 



PAGODELLA. Sw. A sub-genus of Trochus, thus described : 

 " Trochiform ; generally thin, and always not pearlaceous ; aper- 

 ture and pillar perfectly united and entire; operculum horny. 



