BIVALVE SHELLS. 53 



is commenced iinderiiefitli the old one, with a central nucleus 

 which increases by annular additions, and finally has its edges 

 very much thickened and turned upward, giving it a saucer-like 

 appearance, while the old operculum seems as if laying upon the 

 saucer, with its nucleus and some of the adjacent portion 

 projecting over the edge anteriorly. It has in all a diameter of 

 •1 inch. That its form is not due to an individual abnormality 

 is evident from the fact that all the specimens examined were 

 similar." — W. H. Dall, Am. Jour. Conch., vii, 106. 



Mr. Gwyn Jeifreys, in his excellent " British Conchology," 

 records the occurrence, upon the Kentish and Sussex Coasts, of 

 Buccinum widatum, having two or three opercula. " In a bioper- 

 culate specimen, procured by Mr. Rich, one of the opercula is 

 conical and borne on a cylindrical, stalk-like lobe, the other being 

 of the usual shape ; in a second specimen, one operculum is 

 longitudinally oval,with the nucleus nearly terminal (as in Pusus), 

 the fellow operculum being placed at a right-angle to it." 



The duplication of opercula in Bucciium undatwni appears to 

 be paralleled by that of the internal horny pens or shells 

 sometimes occurring in the cephalopod genus Loligo. 



" The above facts," says Dr. Paul Fischer, " modify consider- 

 ably our confidence in specific and generic characters furnished 

 by the operculum. They at least demonstrate that the absence 

 of an operculum will not suffice for the exclusion of mollusks 

 from families the other genera of which are provided with them." 

 Jour, de Conch., 114, 1875. 



The Pelecypoda, or lamellibranchiata differ widely from the 

 two preceding great classes of mollusca in their shells. These 

 are always present, and alwa3^s external, and always bivalve (with 

 sometimes small accessory pieces upon the hinge, as in Pholas). 

 The two great classes of the encephala on the contrary are some- 

 times entirely without a testaceous envelope, and sometimes they 

 possess it internally only ; but in most cases it is present, 

 external, and is usually univalve (in Chiton composed of eight 

 plates). Bivalve mollusca are all aqviatic, and except a very few 

 genera, marine. In genera and species they are much less 

 numerous than the gastropoda, the proportion being about one- 

 third, but in individuals they are relatively much more numerous. 



In their native element the Oyster and Scallop lie on one side, 

 and the lower valve is deeper and more capacious than the 

 upper. Most other bivalves live in an erect position, resting on 

 the edges of their shells, which are of equal size. Those which 

 move about much, like the river-mussel, maintain themselves 

 nearly horizontally, and their keel-shaped foot is adapted for 

 ploughing through sand or mud. 



The bvirrowing species have a strong and stout foot with which 

 they bore vertically into the sea-bed, often to a depth far 



