86 ORDERS AND FAMILIES. 



CYPRINID^E. Shell equi valve, thick, concentrically furrowed or striated; 

 beaks often spiral ; ligament external ; hinge with two or three 

 cardinal teeth in each valve and usually a posterior tooth ; muscular 

 scars oval and distinct, pallial scar entire. Cyprina, 53 ; and 

 Isocardia, 54. 



CYPR^NID^E. Shell suborbicular and thin ; hinge with cardinal and 

 lateral teeth ; ligament external and placed on posterior side of hinge ; 

 foot large and tongue-shaped ; no byssus ; one siphon or two siphons 

 more or less united. Spharium, 81 to 84; and Pisidium, 85 to 89. 



DONACID^S. Shell equivalve, solid, smooth, sub-triangular, or wedge- 

 shaped, inside margin with notches ; ligament external. Donax, 

 104 to 106. 



DREISSENSIID^E. Shell mytiliform, rising to a ridge in the middle of each 

 valve, equivalve, inequilateral, ventricose, beaks anterior ; below the 

 beak a triangular shelf for the anterior muscle ; ligament internal ; 

 hinge toothless or with minute cardinals ; anterior adductor impression 

 small, posterior large ; shell attached by a byssus. Dreissensia, 60. 



ERYCINID^. Shell equivalve, inequilateral, thin ; mantle edges with three 

 apertures ; cartilage internal ; beak calyciform ; foot long, broad, 

 and with a byssus. Kellia, 73, 74 ; Lasaa, 75 ; Lepton, 76 to 79. 



GALEOMMID^E. Shell equilateral, thin ; mantle reflected over a consider- 

 able part of the valves ; mantle edges with three apertures ; foot long, 

 broad and with a byssus. Galeomma, 80. 



GASTROCHCENID^E. Shell equivalve, gaping; with thin, edentulous valves 

 occasionally cemented to a calcareous tube or sheath. The tube is 

 club-shaped, long and slender ; it is covered with adhering particles 

 of sand, and divided off by a partition into two portions, the anterior 

 end containing the shell, the posterior, or narrower end, the siphons. 

 Gastrochana t 159. 



GLYCIMERID^E. Shell more or less equivalve, rhomboidal, gaping and 

 obliquely truncated at posterior end ; hinge, toothless or with two 

 weak cardinals, having an upright ledge to support the ligament, which 

 is external ; pallial scar far in and with a deep sinus ; adductor scars 

 large and conspicuous. Saxicava, 156, 157 ; Panopcea, 158. 



LUCINID.E. Shell equivalve, occasionally toothless ; pallial scar entire ; 

 adductor scars conspicuous, that of the anterior muscle being long 

 and falling within the uninterrupted pallial line. Lucina, 61, 62 ; 

 Loripes, 63, 64 ; Diplodonta, 65 ; Axinus, 66 to 68 ; Montacuta, 69 to 72. 



LYONSIID^. Shell inequivalve, oblong, nearly equilateral ; teeth usually 

 absent, right valve more convex than the left ; hinge with a free plate 

 or ossicle, covering the ligament which is in an internal groove. 

 Lyonsia, 173. 



MACTRID^E. Shell equivalve and triangular; hinge with ligament on 

 larger side of shell in an internal groove, the other portion external ; 

 a forked cardinal tooth in the left valve fitting into a branching tooth 

 in the right valve; muscular scars deep and distinct. Amphidesma, 

 107; and Mactra, 108 to 112. 



