COLLECTION WADE BY W. S AVILLE-K ENT, F.L.S., F.Z.S , ETC. 227 



opinion. It has also assisted in the discovery of certain other 

 characteristic points of distinction between the two forms which 

 have apparently escaped the attention of previous writers. 

 Referring to the two species as Ostrea mordax and 0. glomentta — 

 the former being represented by the usually narrower, more 

 evenly dentated, pink-shelled type, and the latter by the more 

 expanded, irregularly dentate, black-shelled species, — I find that 

 Ostrea mordax is almost invariably attached to its support, and 

 with relation to the contained living animal, by the right valve, 

 the freely moveable or opercular valve being that of the left side. 

 In Ostrea glomerata, on the contrary, the predominating form of 

 attachment is by the left valve, the moveable or opercular valve 

 being that of the right side. From another point of view, 

 recognising the straight or more or less concave border of the 

 oyster's shell as the dorsal, and the opposing or convex border as 

 the ventral edge, it may be represented that the direction of 

 curvature, as compared with the movements of the hands of a 

 watch laid horizontally, has in Ostrea mordax a tendency to 

 describe a right-winding, and in the case of 0. glomerata, a left- 

 winding spire. From a heap of the common market oyster, 

 0. glomerata, it is possible to pick a few examples in which the 

 attachment is by the right valve, the opercular shell being the 

 left one j but these are quite the " exceptions to prove the rule," 

 and as shown in the several bunches here exhibited, and taken 

 hap-hazard from such heaps, the attachment is invariably by the 

 left shell. Similar exceptions may occur also in the case of 

 0. mordax, but the separate method of attachment described 

 being predominant in either instance, this feature may certainly 

 be accepted as an important accessory diagnostic distinction 

 between the two species. It may be further observed of Ostrea 

 mordax that adhesion is almost invariably effected throughout 

 the entire surface of the attached right shell, a circumstance 

 which renders it very difficult to detach these oysters from the 

 rocks without breaking them, and militates against their extensive 

 commercial utilisation. In the common Rock Oyster, 0. glomerata, 

 attachment is usually effected by the basal region or " butt" 



