74 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



protruding between tliem from above. A revision of Ch. sarda ga ve the result that 

 the same arrangement is to be found in that species as well. 



When examining Pseudochama pusilla I found, however, a discrepant state, 

 inasmuch as here the pericardial tubes are covered only on their exteriör sides by 

 the distal sacs, and börder directly on each other in the median line. This differ- 

 ence between Chama and Pseudochama was afterwards found to be constant, the 

 former genus always having the pericardial tubes separated from each other in the 

 median side, the latter having them uncovered on this side. 



In a few cases this rule is, however, somewhat less obvious. Ch. julcesi, when 

 adult, has its pericardial tubes in their foremost part uncovered on the median side, 

 but in the young stage it has them completely embedded. Pseudochama ferruginea, 

 on the contrary, shows in its full-grown stage a slight lobation of the exteriör sacs 

 on the upper sides of the pericardial tubes; the lobes do not, however, penetrate 

 down between these. In a young specimen examined no lobes were observed, and 

 the position of the pericardial tubes was the normal one. 



Besides the differences in position there is also usually some discrepancy in 

 the width and the length of the pericardial tubes, these being in Chama narrower 

 and somewhat shorter and debouch sooner into the exteriör sacs, as we found to be 

 the case in Pseudochama. Even in the histological structure of the distal sacs the 

 genera seem to differ from each other. Chama has large lobes with large bladder- 

 like cells lining them, but in Pseudochama the lobation is somewhat finer and the 

 lining cells are smaller. The state of preservation in the specimens examined did 

 not admit of any sharper analysis of the nature of these histological elements. 



The conditions prevailing in the young specimens of Chama jukesi and Pseudo- 

 chama ferruginea compared with those of the adult stages make it evident that the 

 differences pointed out are rather profound and that aberrations, when present, are 

 of a secondary nature. Which of the two types is the older one phylogenetically 

 it is impossible to stats by comparisons with other Lamellibranchs, as both arrange- 

 ments are common even in primitive forms. The most probable supposition is that 

 both forms have originated from a state such as occurs, for example, in Area (ef. 

 Odhner 1912, p. 299, fig. 2), which seems to include potentiality in both directions. 

 Ontogenetical research is required before this problem can be definitely decided. 



8. Subdivision of tlie recent Chamidae and taxonoinical remarks 



on single species. 



From the analytical studies on the characters of the shells as well as on the 

 anatomical organization presented in the preceding chapters it is evident that these 

 give good grounds on which to base a classification of the recent Chamidae. To 

 revise the whole family in this respect is beyond the purpose of the present treatise, 

 which in spite of the comprehensive material necessary for such a task, will be re- 



