POSTERIOR. 235 



and contracted, but rather short, channel ; spire longer, or as 

 long as the aperture ; exterior folliculated, or tuberculated ; inner 

 lip flattened, as in Purpura ; basal notch small, oblique ; no 

 internal channel; crispata, En. Meth. 419, f. 2. Chem. 187, f. 

 1802. Capilla, Pennant, pi. 72, f. 89, imbricata. Mart. 122. f. 

 1124. ? rugosa. Chem. f. 1473-4/' Sw. p. 305. 



POLYXENES. Montf. A genus of microscopic Foraminifera. 



POMATIA. Gesner. (Gray, Syn. B. M. p. 133.) A genus of the 

 family of tl Cyclostomidse," described as having "an elongated 

 shell with reflexed lips, and a horny spiral operculum." Also a 

 sub-genus of Snails, containing Helix pomatia, Auct. (Gray's 

 Turton, p. 135.) 



PORNUS. Humph. Ampullaria, Lam. 



PORCELLANA. Adanson. Margin ella, Auct. 



PORODRAGUS. Montf. A genus composed of species of Belem- 

 nites, placed by De Blainville in the section characterized as 

 swelled near the apex, and straightened towards the base. 



POSLDONIA. Brong. A genus formed on the cast of a bivalve 

 shell, common on schists from Dillemberg. 



POSTERIOR. {After, behind.) The posterior or hinder part of a 

 bivalve shell, is that in which the siphonal tube of the animal is 

 placed. It is known in the shell, by the direction of the curve 

 in the umbones, which is from the posterior towards the anterior ; 

 also by the ligament, which is always placed on the posterior part 

 of the hinge, when it exists only on one side of the umbones ; 

 and by the sinus (when there is one) in the palleal impression, 

 which is always near the posterior muscular impression. In 

 some shells, however, it is very difficult for a learner to trace 

 these marks ; such bivalves, for instance, as have the ligament 

 spread out on both sides of the umbones ; such as are nearly 

 symmetrical, and have the umbones consequently straight, and a 

 single muscular impression near the centre of the valve. The 

 Brachiopodous bivalves have a different position, with relation to 

 the animal, from the other bivalves, so that the hinge line is the 

 posterior extremity, and the part where the valves open, is the 



