826 I'ULECYPODA. 



Only a few species are very active : Tellina, Yoldia, &c., exe- 

 cute leaping movements by forcibly contracting the foot ; Lascea, 

 Sphcerium, &c.. crawl on immersed bodies or on the surface of the 

 water ; other forms, notably the Pectinidce and Limidce, swim by 

 rapidly opening and closing the valves of the shell ; and some elon- 

 gated forms in which the mantle-edges are fused for a considerable 

 extent swim by forcibly expelling water from the posterior aperture 

 of the mantle (Solen, Solemya). 



There are more than 5,000 living species of Pelecypoda, of which 

 1,000 are Unionidce. They are distributed all over the world, and 

 some marine forms extend to a depth of 2,700 fathoms. 



Fossil forms appear in the Cambrian, and become very numerous 

 in species from the Silurian onwards. Some large groups, such as 

 the Palceoconcha of the Primary and the Rudistce of the Secondary 

 deposits, are quite extinct. 



TERMS EMPLOYED TO DENOTE VARIOUS PARTS OF THE BIVALVE SHELL. 



The umbo, or beak, is the apex of the hollow cone, of which each 

 valve may be regarded as consisting. The apex is usually more or 

 less twisted, sometimes markedly spiral. As a rule, the umbones 

 point forward ; in a few genera, however, they point backward 

 (Nucula). 



An equilateral shell is one in which the umbones are more or less 

 central with regard to its anterior and posterior portion, while in an 

 inequilateral shell the umbones are much nearer one end than the other. 

 On the other hand, equivalve and inequivalve are terms used to express 

 the relation of the two valves to one another as a whole. 



The dorsal margin is adjacent to, the ventral margin opposite to, 

 the mnbones. The anterior and posterior maryim* are respectively 

 the front and hinder edges of the shell. 



The muscles which serve to close the valves leave impressions on 

 the inner surface of each valve. These, when both muscles are present, 

 are known as the anterior and posterior adductor impressions. The 

 impression produced by the muscular edge of the mantle, which curves 

 downwards and backwards from the anterior adductor impression, is 

 known as the pallial line. In shells with only one muscle it is repre- 

 sented by an irregular row of small marks, or it disappears altogether 

 (Ostrea). The pallial sinus is produced by the muscles which retract, 

 the siphons. 



Right and Left Valves. The simplest way of distinguishing the 

 valves as right and left is to hold the shell in such a way that the 

 siphons point towards the observer, and the mouth away from him ; 

 in this position the valve to the right is called the right valve and the 

 valve to the left the left valve. If, however, the animal is not present, 

 it may te remembered that the ligament is nearly always behind the 

 beaka, and that the beaks, as a rule, point forward ; thus the right 



