2 CRETACEOUS PELECYPODA 



Pelecypoda belongs; but they exhibit in the different families a great deal of 

 variation as regards the characters relating to the number of hinge-teeth and the 

 position of the ligament. 



Srd. — The body is slightly irregularly bilateral^ and the two valves are some- 

 what dissimilar and mostly unequal. This is either produced by one of the valves^ 

 becoming attached to a solid object CChamid^, Hippubitid^^ OsTpmD^J, or by 

 the animal being temporarily, or permanently, attached with the aid of certain 

 horny filaments, called the byssus (some Fectinid^J, 



The extent to which the margins of the mantle are united or separated 

 exhibits all stages of gradation. The smallest number of openings in the mantle 

 are two, one anteriorly for the protrusion of the foot, the other posteriorly for the 

 siphons, but sometimes when the latter organs are not especially developed, the 

 mantle is open all round, except dorsally. The edge of the mantle is the only 

 place where occasionally (in some of the Arcidje, Trigoniidje, Fectinid^^ and 

 others) small occelli occur which probably represent the organ of vision. 



The mantle is, as a rule, not firmly connected with the shell, except through 

 the retractile muscles, or adductors, of which there are two,- one anteriorly and one 

 posteriorly,- or only one which in that case is nearly central, and represents, I 

 believe, the posterior muscle. Besides these large retractile muscles there are often 

 smaller lateral muscles developed, which partially support the movements of the 

 gills and palpi, partially the foot, and these smaller muscles are generally attached 

 to the shell below the umbones, or close to the large muscular scars. Only 

 in the family Ltjcinibm, and some other closely allied forms, the outer side 

 of the mantle is partially attached by short muscles to the internal layer of the 

 shell, producing certain very characteristic rugosities. 



The mantle encloses all the internal organs of the animal, or, at least, within 

 it all the other organs can, as a rule, be retracted. 



The mouth is situated always anteriorly close to, a little below and behind, 

 the anterior retractile muscle (if two are present) ; it is provided on either side 

 with a pair of generally sub-trigonal and internally striated labial appendages, 

 the so-called palpi which only in exceptional cases are nearly rudimentary. 

 Internally the mouth does not possess any kind of a solid radula or buccal plates 

 which are usually found characteristic of the Ga&tropods. 



At each side of the body, there is, as a rule, one pair of lamellar gills which 

 are composed of very thin transverse filaments, and these are again connected 

 by fine longitudinal fibres and covered by a very fine ciliated epithelium which 

 keeps the water in constant motion. Along the base, where the principal artery 

 lies, the gills are attached to the body; the remaining part of them is free, but 

 sometimes the leaves on one side are posteriorly grown together with those of 

 the other side. I have examined one or two animals of almost every family of 

 Pelecypoda, and in all I found that there are at least two gill-lamelte present on 

 either side,-never a single one,-though they are often unequal, and one of them 

 sometimes, as in the T:ELLimB^, nearly obsolete. In some Astautibm I observed 



