468 



LAMELLIBRANCHIATA. 



"pallial sinus" (fig. 323, 2). The presence, therefore, of an 

 indented pallial line shows that the animal possessed retractile 

 siphons. In other Bivalves the respiratory siphons are of 

 small size, . and are destitute of retractor muscles, so that 

 they cannot be withdrawn within the shell. In these cases 

 the " pallial line," or the impression caused by the attach- 

 ment of the muscular border of the mantle, is unbroken in 

 its curvature, and presents no indentation (fig. 323, 1). In 

 another group of the Bivalves there are no respiratory 

 siphons at all, and the mantle-lobes are free, and are not 

 united to one another at their edges. In these cases also, 



Fig 323. Shells of Lamellibranchiata. 1, Cyclas amnica, a dimyary shell with an entire 

 pallial line. 2, Tapes pullastra, a dimyary shell with an indented pallial line. 3, Perna 

 ephippium, a monomyary shell (after Woodward), a, Pallial line ; b, Muscular impres- 

 sions left by the adductors ; c, Siphonal impression. 



the pallial line is unbroken or " simple." When, therefore, 

 we find a Bivalve shell in which the pallial line is not in- 

 dented by a sinus, we know that the animal which inhabited 

 the shell either possessed no siphons, or that if siphons were 

 present, they were small and not retractile. 



In accordance with these considerations, the Lamellibranchi- 

 ata are divided into two sections, according as respiratory 

 siphons are present or absent, and according to their nature 

 when they exist. 



SECTION A. ASIPHONIDA. Animal without respiratory 

 siphons ; mantle-lobes free ; the pallial line simple and not 

 indented (Integro-pallialia). 



This section comprises the families Ostreidce, Aviculidw, 

 Mytilidce, Arcadce, Trigoniadce, and Unionidce. 



SECTION B. SIPHONIDA. Animal with respiratory siphons ; 

 mantle-lobes more or less united. 



