44 OKHAMANDAL MARINE ZOOLOGY REPORT 



The general characters of the family Anomiidae, which comprises Anomia and 

 Placuna as its two distinctive genera, may be summarised as follows : 



The organs exhibit a very high degree of asymmetry and the shell is distinctly 

 inequivalve. The valves show a pronounced tendency to assume an orbicular outline, 

 and are very much flattened or compressed laterally : the right valve is almost flat, the 

 left weakly convex. 



In young specimens the shell is thin and more or less translucent ; the periostracum 

 is very delicate, and disappears soon after formation. Pigmentation of the adult shell 

 is rare, though it is common among young forms to show a beautiful satiny pink or 

 yellowish lustre. 



The mantle edges are free and siphons are absent. A single adductor muscle (the 

 posterior) is present, large, and situated sub-centrally. The foot is fairly well developed, 

 either tongue-shaped or cylindrical. In Anomia the byssus is modified into a large and 

 important calcified organ serving for fixation ; in Placuna a byssus is absent, even in 

 the very young. 



The visceral mass is asymmetric, the right aspect of most of the organs being 

 developed at the expense of the left. The crystalline style is lodged almost entirely in 

 the right mantle, and practically the whole of the reproductive glands are likewise in 

 intimate association therewith. 



The intestine is short, and lies posterior to the stomach. The crystalline style is 

 of enormous relative length. Special sense organs, other than tactile, are little 

 specialised. The heart lies dorsal to the rectum and projects freely into the pallial 

 cavity : no pericardium is present, and the ccelomic cavity is reduced to insignificant 

 or vestigial dimensions. There is a single aorta the anterior. 



The genital glands open into the kidneys. The sexes are separate as in the true 

 pearl oysters (Margaritifera spp.). 



The two ctenidia are smooth, with simple reflected tubular filaments, and are 

 organically fused together along the median line ; each has a supplementary (or fifth) 

 external lamella which is free and unattached to the mantle. The filaments are parallel 

 and practically free, there being neither interlamellar nor interfilamentary organic 

 unions ; cohesion is effected by means of interlocking cilia situated on the opposing 

 lateral faces of successive filaments : in Placuna certain of these ciliated areas are 

 developed into true "ciliated discs" ; in Anomia the opposing cilia interlock, but are 

 not specialised into " discs." 



The genera of the family Anomiidas, as already pointed out by Pelseneer, by the 



ensemble of their organisation, are among the most archaic of existing Lamellibranchs 



notably in the simple structure of the branchiae, in the position of the heart dorsal 



to the rectum, in the muscular structure of the auricles and in the absence of a 



posterior aorta. 



With these persistent archaic characters are associated several curious and highly- 



