M iscella nea. 185 



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

 mere inspection of any large number of siiecimens. 



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

 and afiFords 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.P/ac?mfl,sp. Tj^mk.^EpUppiumjChemw.; Placenta B, Schum. 

 Shell purplish, subopahe ; 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, C\i.emx\. C viii. t. 79, f. 714. cop, 

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



Placenta Ephippium, Retz, 1788. 

 Inhab. China, India. 

 B. 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. IT. Placenta; Placenta, Schum. Shell semi-transparent, flat, 

 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. 1154 ; Chemn. Conch, viii. t. 79. f. 176. 

 cop. E.M. t. 173. f. 2. 



Shell colourless, semi-transparent ; when young, pale purplish. 



Inhab. China. N.W. Coast of Australia ; Earl of Derby. Port 

 Essiugton. 



The shells vary a little in the inequality of the hinge-ridges, but 

 the hinder is always the longest. 



I may remark that Chemnitz gives the best character for the spe- 

 cies, and has observed the character furnished by the hinge, which 

 has been overlooked by Lamarck, and, as far as I am aware, by all 

 recent authors. 



