228 PAILEAL IMPB.ESSIOJS". 



within so as to form a sort of ledge upon which the operculum 



rests. AmpuUaria 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. PL xxv. 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. 

 P^CILASMA. Darwin. Cirrip. P. 99, including Teilasmis, 

 Hinds. A genus of Pedunculated Cirripedes, the shell of which 

 is thus described, "Valves 3, 5, or 7 proximate ; carina extend- 

 ing only to the basal points of the terga ; with its lower end 

 either truncated or produced into a deeply imbedded disc, scuta 

 nearly oval, with their umbones at the vertical angle." 

 "PADOLLUS. Montf. 1810. A genus composed of species of 

 Haliotis, with a strongly marked spiral groove. Ex. H, tri- 

 costalis, Lam. PL xv. fig. 339. 

 PAGODELLA. Swainson. Should be Pagodtjs, Gray. A sub- 

 genus of Trochus, thus described : " Trochiform ; generally thin, 

 and always not pearlaceous ; aperture and pillar perfectly united 

 . and entire ; operculum horny. P. major. Mart. 163. f. 1541, 



1542. P. tectum. P. persicum. lb. f. 1543, 1544," Sw. p. 351. 

 PAGODTJS. Gray, 1839. See PAaoDELLA. 

 PALLEAL IMPEESSION. {Pallium, a mantle.) The mark 

 or groove formed in a bivalve shell by the muscular attachment 

 of the mantle, which, being always found near the margin of the 

 shell, is sometimes termed the marginal impression. In bivalves 

 with two muscular impressions it passes from one to the other. 

 If in passing, it takes a bend inwards posteriorly, it is said to 

 be sinuated, and that part is called by Mr. Gray, the Siphonal 



