88 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



doubtful; one cannot say whether it is a primitive character or a product of spe- 

 cialization. In any case it, together with the blood system, helps to give the 

 Chamidae a separate position among the existing LameUibranchia. 



The opinion criticized above as to the relations between Chamidae and Cardi- 

 idae has proved to have no solid basis. If novv we examine other suppositions ex- 

 pressed in the literature we find two views based on the external sculpture of the 

 shell, namely that of Fischer about the agreement between the nepionic stage of 

 Ecliinochama and Venerupis, and Boehm's assumption of its greater resemblance to 

 Astarte. Both opinions are based exclusively on the sculpture of the shell, without 

 any attempt to find support for them from anatomy. Even the view about their 

 origin from Carditian forms set forth by Dall and based on the structure of the 

 hinge does not take into account agreement in anatomy. 



A comparison of the Chamidae on the one hand with the three families men- 

 tioned on the other in regard to anatomy does not support either theory. Astartidae 

 and Carditidae have a completely open mantle, Veneridae, on the other hand, well 

 developed siphons, and Chamidae occupy a medial position between these extremes 

 as far as the siphonal openings are concerned. As to Cardita and Astarte these forms 

 are in many respects more simply organized than the Chamidae and therefore might 

 represent in these points earlier stages than the latter (e. g. the gills) ; but in other 

 characteristics they show a peculiar specialization (e. g. the spiral coecum on the 

 ventral side of the stomach in certain forms of Cardita; ef. Pelseneer 1911, p. 38; 

 pl. XIII, fig. 2). But if we consider the unplicated synaptorhabdic nature of their 

 gills and recall to mind the above conclusions on the phylogeny of the respective 

 gill types we must at once reject the possibility of a closer relation between Chamidae 

 and Astartidae — Carditidae. 



As to the origin from the Veneridae this supposition seems to be more probable. 

 Their gills are plicated but otherwise they are more primitive than those of Chamidae 

 on account of the separation of the efferent filamentar vessels, quite as in Cardiidae. 

 On the other hand the Chamidae represent, as pointed out above, a more primitive 

 stage of consolidation on account of the individual variation of the gill fixation. In 

 the inner organization agreements exist between the two familiae in the simplicity 

 of the duodenum and the absence of a separate crystalline coecum, but differences 

 in the coiled course of the intestine of Veneridae (e. g. Circe; ef. Pelseneer 1911). 

 Of the rest of the inner organization of Veneridae no detailed accounts are given. 



Thus we find primitivity in the Chamidae on the one hand and in the Ve- 

 neridae and the Cardiidae on the other. These appearances make it probable that 

 these families might have a common origin. 



We now return to the characteristics of the shell in order to test their evi- 

 dence as to the mutual relations of the forms in question. 



Is should be remarked that Fischer's and Boehm's opinions on the affinities 

 of Chamidae ref er only to Echinochama and thereby al so to Pseudochama in a wider 

 sense according to the argument in chapter 2 of the present work, and that An- 

 thony speaks of the anatomy of Chama proper. We are then justified in asking 



