KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3. 75 



stricted to giving some chief points and to drawing attention to the possibility and 

 the advantage of using even the soft parts of the animals for the classification. It 

 is to be hoped that the views given here will be noted by subsequent writers on 

 this subject. 



For the discrimination of different species \ve really have found available marks 

 in the external morphology, for example the relative size and extension of the ad- 

 ductors, the mantle slits, siphons, labial palps, foot and other characteristics. For a 

 subdivision of the family into large groups, on the other hand, the inner anatomy 

 as well as the hinge construction offer good and suitable grounds. These justify 

 the classification of the Chamidae given below. According to our present knowledge 

 the family comprises only two recent genera, one of them including three subgenera, 

 which may be characterized and arranged thus: 



I. Shell with the apices twisted to the right and the left valve attached to the 

 substratum; nepionic shell verv small (about 0,5 mm), with concentric and 

 radial sculpture; lateral teeth persistent: no marginal teeth of the adult shell; 



2(a + b) 4 b 6 b II 



dentition -. Animal with a finger-shaped coecal appendage 



3a 1 3b 5b I 



projecting forward from the left side of the stomach; nephridia with the peri- 

 cardial tubes entirely embedded in the distal sacs Chama Linné 1758. 



II. Shell with the apices twisted to the left and the right valve attached ; nepionic 

 shell distinct; adult shell with a left marginal tooth. Animal without lateral 

 coecal appendage of the stomach; nephridia with the pericardial tubes not 

 covered on their median side by the distal sacs Pseadochama n. gen. 



1. Teeth 1 and 3 b of uniform size, 2 b distinct, 4 b of moderate size. LP II 



2 a 2 b 4 b M II 



persistent, though vestigial; dentition of the adult shell - — • 



1 3b 5b i 



Nepionic shell (about 2 mm in length) with concentric and radiating sculp- 

 ture. Alimentary canal with an indication of a separating crystalline sac; 

 no posterior coecum of the stomach subg. Eoyseuma n. subg. 



2. Teeth 1 and 4 b highly developed, 3 b and 2 b obsolete. LP II obliterated 



2 a 2 b 4 b IM 



in the adult shell; dentition - -. Nepionic shell with con- 



1 3 b 5b| i 



centric sculpture only. Alimentary canal with the crystalline sac not diffe- 

 rentiated from the duodenum. Stomach with a distinct posterior coecum. 



A. Shell without lunula, inequivalve, always attached, never spiny, usually 

 smooth, squamous or laminated. Nepionic shell suborbicular, small- 

 sized (about 1,4 mm) subg. Pseudochama s. str. 



B. Shell with distinct lunula, nearly regular and equivalve, attached when 

 young, usually free when adult, ribbed and spiny. Nepionic shell largé, 

 elongated (about 2,4 mm) subg. Echinochama Fischer 1887. 



