CHAPTER X. 



ORDERS AND FAMILIES, 



FOR the purposes of easy reference, we have here adopted 

 alphabetical order, and not systematic order, as in the pre- 

 ceding chapter. The descriptions of the families are such as are, 

 it is hoped, sufficient to distinguish them among their British repre- 

 sentatives. The numbers, as in all the chapters, refer to the 

 coloured plates. 



Amphineura. (GASTROPODA.) Plate xvi. Nos. 190 to 200. 



. Shell somewhat resembling that of a wood-louse, and com- 

 posed of eight overlapping valves surrounded and kept in position by 

 a muscular girdle, external portion of the valves largely composed 

 of chitin. Chiton, 190 to 200. 



Eulamellibranehiata. (PELECYPODA.) Plates v. to xv. 



Nos. 49 to 178. 



ANATINID/E. Shell thin, oval or oblong, gaping on each side, and trun- 

 cated at posterior end ; internal cartilage contained in a receptacle 

 beneath the beak in each valve ; hinge with free crescentic ossicle ; 

 muscular scars small and irregular, pallial scar narrowly but deeply 

 sinuated ; a fourth pallial orifice. Thracia, 174 to 178. 



ASTARTIDJE. Shell triangular, thick, concentrically grooved or furrowed; 

 ligament external ; hinge with three cardinals in each valve, and with 

 two laterals in each valve that are ridge-like and obscure. Astarte, 

 50 to 52. 



CARDIKUE. Shell equi valve and heart-shaped, with radiating ribs, and 

 often with concentric foliations forming scales and spines and 

 tubercles ; beaks prominent ; ligament external ; the two adductor 

 scars oval and distinct, pallial scar entire ; no byssus ; foot long and 

 sickle-shaped. Cardium, 129 to 138. 



CARDITID^. Shell equivalve and inequilateral ; ligament external ; 

 muscular scars large, round, and deep, pallial scar entire ; hinge with 

 a lateral tooth in both valves, one double cardinal tooth in the right 

 valve, and two cardinals in the left. Cyamium, 49. 



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