KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 52. N:0O IG. 3 
denomination (by MUNIER-CHALMAS) in opposition to the rule of homology. This 
comprehension of the genus Pseudochama as representing a phenomenon of invers- 
ion of the Chama type has been uncritically accepted by all the malacologists writing 
on the subject, and has hitherto rested unrefuted. 
After a close investigation of many forms of Chamas, considering their onto- 
geny as well as the conchological and the anatomical conditions, I was convinced 
that a division into the two genera Chama and Pseudochama (comprising also 
Echinochama) is necessary. The distinguishing characteristics are the following: 
The dentition in Chama may be expressed (in accordance with BERNARD, 1895) thus: 
: 9 le ST 
left valve: b = Ab 4b . RN That of Pseudochama has the formula: 
right valve: | 3a i 3b (El 
The difference is more obvious in young than in 
| 
leftivalvesov| 2av2b; 4bob II 
Hahtvalvetsb dolt 3biox lh & 
older shells. The nepionic shell in Chama is very small (about 0.5 mm) and has a minute 
sculpture of close radiating and somewhat more distant concentric riblets (cf. AN- 
THONY 1905). In Pseudochama the nepionic shell is sculptured just as in Hchinochama 
with rather remote concentric lamellae, and no or only traces of radiating riblets; 
further its size is more considerable (1.4—2.5 mm). There are also some points of 
difference in the anatomical characteristics, inasmuch as Chama has a stomachal 
coecum directed forewards, which is wanting in Pseudochama, and the nephridia of 
the former genus have the pericardial tubes wholly embedded in the external sacs, 
while in Pseudochama they are left uncovered on their median side. 
Survey of the Anatomy of Chamidae. 
The following accounts are based on an examination of the forms present in 
the collections from N. W. Australia and are an extract from a more exhaustive 
inquiry on the family, which will be published later on. As they are of importance 
for the comprehension of affinities in the group of Lamellibranchia and have a spec- 
ial interest in the matter of a comparison with Chamostrea described later, they 
are included in this publication. 
The gills and the blood system. When the respiratory organs of the forms 
of Lamellibranchia have been studied, the examination has always been limited to 
the detailed anatomy and the histology. But such an investigation does not lead to 
an exact comprehension of the organization of the branchial apparatus as a whole. 
Its function is full-filled in the service of the circulatory system, and therefore we 
have to study it as an organ intimately connected with the blood system, a point 
of view which has been too much neglected. The accounts accessible in the liter- 
ature on the subject give very little and seemingly contradictory information about 
the gills as blood-carrying organs. I therefore made it my task, by studying the Cham- 
idae, to get an idea of the whole blood circuit within the gill. This was possible by 
following, on sections, the blood vessels back to the larger trunks from where they 
