Zoological Society. 151 



top of the outer side. It agrees however very nearly with the de- 

 scription and figure given by the latter naturalist from the specimen 

 found by him near Dover, and there can be no doubt of its identity 

 with his specimen of Vespertilio emarginatus . 



2. On the Species of the Genus Placenta of Retzius. 

 By J.E. Gray, Esq., F.R.S. etc etc. 



Lamarck describes three species of this genus, depending on the 

 general outline and the waved or flat form of the shell, characters 

 which are liable to considerable variations, as may be found on the 

 mere inspection of any large number of specimens. 



I have observed that the hinge forms a more permanent character, 

 and affords the means of dividing the species into two sections, and 

 furnishes characters which separate them from each other. In both 

 subgenera the right valve is the flattest, and bears the ridges of the 

 hinge. 



Sect. I. Placuna, STp.La.mk.=Ephippium,Chemn. ; Placenta fi, Schum. 

 Shell purplish, subopake ; hinge-ridges rapidly diverging from one 

 another at about the angle of 45 degrees. Muscular scar under 

 the centre of the hinge. The ridges of nearly equal length. 



1. Placenta Sella. — Shell flexuous, outline rather rhombic, being 

 straight in front and rather notched behind ; the ridges of the hinge 

 not longer than they are separate from each other at the base. 



Anomia Sella, Gmelin, S. N. 3345, 1788. 

 Placuna Sella, Lamk. Hist. N. 2. 



Ephippium anglicanum maximum, Chemn. C. viii. t. 79. f. 714. cop. 

 E. M. t. 174. f. 1. 



Placenta Ephippium, Retz. 1788. 

 Inhab. China, India. 

 fi. Shell nearly flat, subquadrangular. 

 Inhab. Australia. Brit. Mus. 



2. Placenta papyracea ; Placuna papyracea, Lamk. Hist. N. 2 = 

 Ephippium parvum, Chemn. Conch, viii. t. 79. f. 719. cop. E. M. 

 t. 174. f. 2. 



3. Placenta Lincolnii. — Shell flat, outline suborbicular, rounder 

 before and behind ; ridges of the hinge elongate, longer than they are 

 separate from each other at the base. 



Inhab. Australia ; Mr. W. Davison. British Museum. 

 I wish to name this species after my excellent friend Mr. Abraham 

 Lincoln, who kindly presented me with the specimen here described, 

 and who is well known for his fondness for conchology and the libe- 

 rality with which he allows persons to use his extensive collection. 

 Sect. II. Placenta; Placenta, Schum. Shell semitransparent, fat, 

 outline suborbicular ; ridges of the hinge very gradually diverging 

 from each other, the hinder ridge much the longest. Muscular scar 

 rather in front of the middle of the hinge. 



1 . Placenta orbicularis, Retz. ; Placuna placenta, Lamk. Hist. N. 3 ; 

 Anomia placenta, Linn. S. N. 1 154 ; Chemn. Conch, viii. t. 79. f. 1 76. 

 cop. E.xM. t. 173. f. 2. 



