54 OKHAMANDAL MARINE ZOOLOGY REPORT 



central point in the hinge line. They are long, narrow, and greatly compressed 

 laterally ; they run ventrally, diverging as they go. The posterior is considerably 

 the longer of the two, measuring over 2 cms. in fully-grown shells, whereas the anterior 

 is but 1'5 cms. long. These teeth have their free edges slightly rolled or curved 

 outwards upon themselves in such a way as to give the ligament a very secure grip 

 (Plate II., fig. 1). 



A number of minute teeth occur on each side of the cardinal projections and 

 parallel with them, so that those in front of the anterior cardinal tooth run anteriorly 

 and ventrally, and those behind the posterior cardinal, towards the posterior and 

 ventral aspects. Frequently one, or, less frequently, two minute teeth occur between 

 the cardinals. 



All these small hinge teeth alternate with papilliform terminations along the 

 dorsal aspect of the mantle ; in young specimens, up to 2 cms. long, they are absent. 



In the three spaces or pockets formed by the cardinal teeth and their ligaments, 

 lie three corresponding dorsal portions of the mantle. 



The internal surface of the valves is smooth, with a high polish. The impression 

 of the single adductor muscle is situated sub-centrally, a trifle dorsal to the centre, 

 and is very noticeable. The minute insertion scar of the single pedal levator muscle 

 may usually be seen upon the left valve, a little posterior to the ventral extremity of 

 the anterior cardinal hinge ligament. 



No well marked pallial line, such as we have in Cardium and Mactra, nor a 

 regular series of scars caused by the insertion of the pallial muscle bundles, as seen in 

 the true pearl oyster (Margaritifera vulgaris), can be distinguished. The only vestiges 

 of such are two or three narrow and elongated faint scar-impressions, a little anterior to 

 the antero-ventral corner of the adductor scar, and two others of smaller size at a 

 corresponding level behind it. In adult shells the map of the pallial muscles is 

 conspicuous as a fan-like radiation of shallow grooves ; all the main bundles have 

 their corresponding impressions on the inner surface of each valve. 



The shell in young and immature individuals is very thin, transparent, and usually 

 colourless, whence comes its popular name of window-pane oyster. When fully grown 

 the shell attains a thickness of about one millimetre, and losing its transparence 

 becomes sub-opaque, white, and somewhat friable, suggestive of dull white mica which 

 has lost its transparency through weathering. 



The substance of the shell is composed of the same three layers as are characteristic 

 of most Lamellibranch shells, but here the organic basis is unusually well developed, 

 while the periostracum, or superficial horny layer (seen well in Unio and Area], is 

 exceedingly thin and scarcely discernible even in young specimens, where it can 

 scarcely yet have been worn off by attrition or decomposition. In old specimens the 

 middle- or prismatic layer becomes actually superficial, although the mud which lodges 

 iu. 'the roughness of the outer surface of the middle layer gives, as already mentioned, 



