234 POI.YTROPA. 



which have large continuous costse, so as to present the appear- 

 ance of many-sided shells. T. polygonus, fig. 383. This generic 

 name may be used to include aU those species of Turbinella, 

 Auct. which have very small folds on the columella. 



POLYGYRA. Say. A genus of Heliciform shells, characterized 

 by the large number of close set whorls, constituting the spire. 

 Ex. P. Septemvolvus, fig. 2/5, 276. 



POLYLEPAS. Bl. (UoXvq, polus, many; XeTrae, lepas, Linn.) 



ScALPELLUM, Auct. 



POLYMORPHINA. D'Orb. A genus of microscopic Forami- 

 nifera. 



POLYPHEMUS. Montf. A genus composed of species of Acha- 

 tina, Auct. which have elongated apertures, short spires, and 

 an undulation in the outer lip. P. Glans, fig. 288. 



POLYPLAXIPHORA. Bl. The second class of the sub-type 

 Malentozoa, Bl. containing the genus Chiton. 



POLYSTOMELLA. Lam. A genus of microscopic Foraminifera. 



POLYTHALAMACEA. Bl. (UoXvg, polus, many ; OaXajuoe, tha- 

 lamos, chambers.) The third order of Cephalophora, Bl. the 

 shells of which are described as straight, more or less symmetri- 

 cally convolute, divided into several chambers. The septa are 

 sometimes, but not always, pierced by one or more siphons. 

 This order is divided into the families, Orthocerata, Lituacea, 

 Cristacea, Ammonacea, Nautilacea, Turbinacea, Turriculacea, all 

 of which contain genera of chambered shells. De Blainville 

 arranges these families according to the degree in which the 

 spires revolve. The first being straight, as the Orthocerata, and 

 the last being so closely coiled up, that the last whorl covers the 

 rest, as in the Nautilacea. 



POLYTHALAMIA. Lam. The first division of the order Cepha- 

 lopoda, Lam. containing the following families of chambered 

 shells, viz. Orthocerata, Lituacea, Cristacea, Sphserulacea, Radio- 

 lata, Nautilacea, Ammonacea. Fig. 463 to 484. 



POLYTROPA. Sw. A genus of " Scolyminee," Sw. thus described: 

 " Bucciniform ; but the base narrow, and ending in a straight 



