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, Esa., F.R.S. eve. 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, sp.Lamk.= Ephippium,Chemn.; Placenta 3, 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. 
fp. 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. 
tol74. £52. 
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, flat, 
outline suborbicular ; ridges of the hinge very gradually diverging 
Srom 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. 1154; Chemn. Conch. vii. t. 79. f. 176. 
cop. E. M. t. 173. f. 2. 
