238 PTEROPODA. 



This genus differs from Tellina in not having a posterior fold in 

 the margin. Fig. 100. The species are found in temperate and 

 tropical climates. 



PSAMMOCOLA. Bl. (^afijiog, psammos, sand ; cola, an in- 

 habitant.) A name given by De Blainville to shells of the genus 

 Psammobia, including Psammot^a of Lamarck. 



PSAMMOT^A. See Lam. Psammobia. 



PSEUDOLIVA. Sw. A genus of " EburninEe," Sw. thus described : 

 "Shell thick, oval, oliviform, veDtricose; spire very short, acute ; 

 base with two parallel grooves, one of which forms a notch at the 

 base of the outer lip ; suture slightly channelled ; inner lip very 

 thick, and turning inwards ; aperture with an internal canal. Con- 

 nects the Turbinellid^; with the Voltjtid^e. P. plumbea, 

 Chem. 188. f. 1806, 1807." Sw. p. 306. 



PSILOSTOMATA. Bl. The third family of Aporobranchiata, Bl. 

 containing no genera of shells. 



PTEROCERAS. Auct. (Urepop, pteron, a wing ; icepag, ceras, horn.) 

 Fam Ailees, Lam. — Descr^ Turrited, oval, ventricose, thick, tuber- 

 culated ; spire short ; aperture oval, terminating in a lengthened 

 canal at both extremities ; outer lip thickened, expanded, produced 

 into horn-shaped, hollow, thickened spires, with an anterior 

 sinus apart from the canal. — Obs. This genus, containing the 

 shells commonly called Devil's Claws, Gouty Scorpions, Spiders, 

 &c. is distinguished from Strombus by the digitations of the outer 

 lip. No fossil species are known. Fig. 405, P. aurantiaca. 



PTEROCYCLOS. Benson. Syn. B. M. p. 133. A genus formed of 

 species of Cyclostoma, Auct. which have "a groove or hole at the 

 hinder part of the mouth." 



PTEROPODA. Lam. (7rrepov, pteron, a wing ; irovg, pom, a foot.) 

 The first order of the class Mollusca, Lam. consisting of molluscs 

 whose organs of locomotion consist of a pair of wing-shaped fins. 

 This order contains the genera Hyalsea, Clio, Cleodora, Spiratella, 

 Cymbulia, and Pneumoderma. To which may be added other 

 genera enumerated in explanation of figures 220 to 226. They 

 may be thus distinguished. 



1. Atlanta. Shaped like Nautilus, symmetrical. Fig. 220. 



