PULVINITES. 239 



2. Spiratella. Spiral, not symmetrical. Fig. 224. 



3. Creseis. Straight, thorn-shaped. Fig. 222. 



4. Vaginula. Straight, widened in the centre ; apex pointed. 



Fig. 225. 



5. Cuvieria. The same ; apex blunt. Fig. 223. 



6. Cleodora. Aperture with three spines; apex recurved. 



Fig. 221. ' 



7. Hyal^a. Vaulted, open extremity, three-cornered ; apex 



tridentate. Fig. 226. 



PTEROPODA. Bl. The second family of Nucleobranchiata, Bl. 

 the shells of which are described as symmetrical, extremely thin, 

 transparent, longitudinally enrolled, either forwards or backwards. 

 The animals are remarkable for a pah' of broad, flat, natatory 

 organs or membranaceous fins, from which the family derives its 

 name. It contains, in the system of De Blainville, the genera 

 Atlanta, Spiratella, and Argonauta, to which may probably be 

 added Pharetrium, Konig ; Entalis, Defrance. 



PULLASTRA. Sow. Fam. Conques Marines, Lam.— Descr. Equi- 

 valve, ovate or oblong, transverse, inequilateral ; hinge with three 

 diverging cardinal teeth in each valve, notched at the termina- 

 tions ; muscular impressions two in each valve ; palleal impression 

 having a large sinus ; ligament external, partly hidden by the dorsal 

 margin. — Obs. This genus includes the Venerirupes of Lamarck, 

 and several species of his Veneres, they are found in the sand on the 

 shores of temperate and tropical climates. Fig. 120, P. textile. 



PULMONOBRANCHIATA. Bl. The first order of the first section 

 of Paracephalophora monoica, containing the families Limnacea, 

 Auriculacea, and Limacinea. 



PULVINITES. Defr. (Pulvinus, a cushion.) Fam. Malleacea, Lam. 

 — Descr. Sub-equivalve, inequilateral, compressed, thin, slightly 

 gaping posteriorly ; one valve flat, the other rather concave ; hinge 

 linear, short, divided into perpendicular grooves ; muscular impres- 

 sions two, one sub-central, the other above it, nearer the hinge. 

 — Obs. This fossil shell is imperfectly known, and it is difficult to 

 give a sufficient reason for separating it from Perna. It comes 

 from the Baculite limestone of Normandy. Fig. 170, P. Adansonii. 



