76 PALUDINA. 
tremity ; outer-lip crenulated, inflected; inner-lip smooth, callous 
towards the spiral extremity ; dorsal area wide, sometimes indis- 
tinctly marked. Obs. The Ovuli were placed by Linneeus in 
his genus Bulla, from which they are very remote. They differ 
from Cypreea in having the inner-lip smooth. We have given 
representations of their different forms as follows: O. ovum, 
fig. 440. O. verrucosum (Calpurnus, Montf.) fig. 441. O. Volva, 
the weaver’s shuttle, (Radius, Montf.) fig. 442; and O. gibbosum, 
(Ultimus, Montf.) fig. 443. 
OXYSTOM ATA. Bl. The fifth family of Asiphonibranchiata, 
Bl. containing the genus Janthina. 
PACHYMY’A. Sow. (mayés, pachos, thick, and Mya.) Fam, 
Cardiacea? Lam. Descr. Obliquely elongated, equivalve, thick, 
sub-bilobed, with beaks near the anterior extremity; ligament 
partly immersed; attached to prominent fulcra. Obs. This 
singular fossil is shaped like Modiola, but the shell being ex- 
tremely thick and the ligament attached to a prominent fulcrum, 
it is dificult to know where to place it. Fig. 130, Pachymya 
Gigas. 
PAC’LITES. Montf. A genus composed of species of Belemnites, 
Auct. described as curved towards the extremity with a pore at 
the apex and a straight lengthened aperture. Hx. B. ungulatus, 
Bl. 
PADOL’‘LUS. Montf. A genus composed of species of Hatioris, 
with a strongly marked spiral groove. Hz. Haliotis tricostalis, 
Lam. Fig. 339. 
PA’LLEAL IMPRESSION. (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, issometimes termed the marginalimpression. In bivalves 
with two muscular impressions it passes from one to the other. 
If in passing it takes a bend inwards posteriorly, it issaid to be 
sinuated, and that part is called by Mr. Gray the Siphonal scar. 
PALLIOBRANCHIA’TA. Bl. The first order of the class 
Acephalophora, Bl. Containing in the first section the symme- 
trical bivalves Lingula, Terebratula, Thecidea, Strophomena, 
Plagiostoma, Dianchora and Podopsis; in the second section, 
the genera Orbicula and Crania. 
PALMINA. Gray. Differing from Orion, in having but one 
auricle. 
PALUDINA. Lam. Fam. Peristomata, Lam. Cricostomata, Bl. 
Descr. Varying in form from oval to globose, in some instances 
oblong, covered with a greenish horny epidermis; spire acute, 
composed of rounded whorls; aperture ovate; peritreme entire, 
slightly modified by the last whorl; operculum horny, con- 
centric. Obs. The construction of the operculum distinguishes 
this genus of fresh-water shells from Valvata and Cyclostoma. 
The Paludine are viviparous. Fig. 321, P. Achatina. 
