KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59- N:0 3. 87 



the anterior laterals are absent in that genus, a condition coramon with Chama, but 

 its hinge is of a somewhat different type, having in the right valve 2 cardinals and 

 2 laterals, in the left 1 tooth of each kind. Further, Lithocardium is an eocene 

 fossil, and the Chamidae appear already in the lower cretaceous system, though it 

 may be doubted whether the forms described from these formations are referable to 

 the recent genera. 



Thus neither the anatomical nor the conchological evidence for Anthony's 

 opinion about an origin from Cardiidae is verv convincing. It seems to me that he 

 bas been fascinated by Pelseneer's authority, when this writer suggests a close 

 relation of the familiae in question because of the structure of the gills. But in 

 this respect the Chamidae are not more closely related to Cardiidae than they are, 

 for example, to Veneridae (ef. Johnstone 1899 and Ridewood 1903). And if there 

 were agreement in the structure of the gills, their external morphology is against 

 such an opinion because of the fact mentioned above that in Chamidae the gills 

 exhibit a more primitive stage in the individual variation of their fixation to the 

 body and to each other. In this respect Cardium has proceeded to generic con- 

 stancy. It must be admitted, however, that the anatomical structure of the gills 

 of Cardium is somewhat more primitive on account of the separated efferent fila- 

 mentar vessels (ef. Johnstone 1899). 



There is a fact not mentioned by Anthony nor by Grieser, who also accepts 

 the opinion of the former author, that gives the anatomy of the Chamidae a special 

 peculiarity and separates these forms from other Lamellibranchs that have been in- 

 vestigated with regard to the matter. This is the structure of the arterial blood system, 

 above all the course of the arteria gastrica (or visceralis). In Chamidae this erna- 

 nates immediately from the sinus in front of the pericard, and then goes forward to 

 the back side of the stomach. This specialization is not recorded for any other 

 Lamellibranch, though the circulatory system is studied comparatively by Ménégaux 

 (even with regard to the Chamidae, where he did not, however, observe this peculi- 

 arity), and is described by many authors for many different forms (ef. Egger 1887, 

 Pholadidea, Jouannetia; Johnstone 1899, Cardium; Vlés 1909, Mya; Schwanecke 

 1913, Anodonia, and others). In all these forms the arteria visceralis arises as far 

 towards the front region as the vicinity of the mouth. In all probability the arteria 

 visceralis is homologous with the posterior aorta (also called arteria visceralis) in the 

 gastropoda, which separates from the anterior or aorta cephalica immediately after 

 the departure from the ventricle. In Chamidae the same organization prevails, which 

 may perhaps be a sign of primitivity. The aberration observed in Ch. sarda, where 

 a small collateral runs from the oesophageal trunk of the aorta to the stomach, 

 may then be considered as representing the first step in the development of an 

 arrangement like that of the Lamellibranchia in general. 



Another peculiarity in the anatomy of Chamidae is the presence of a buccal 

 centre in the nervous system, detected by Grieser. A similar feature is known 

 only in Phaseolicama magellanica (Igel 1907), though sought for in vain by Pelse- 

 neer (1911) in the same species. Its nature, under such circumstances, is very 



