18 COMPARATIVE ANATOMY CHAP. 



Sub-Order 2. Tellinaceae. 



Dimyarian with completely separate siphons. Foot large. Gills smooth. Fam. 

 Tellinidse (Tellina). Fam. Donacidae (Donax), Mactridae (Mactra). 



Sub-Order 3. Veneracea. 



Dimyarian with somewhat folded branchial lamellae. Siphons separate, and foot 

 large. Fam. Veneridae (Venus, Meretrix [Cytherea], Tapes). Fam. Petricolidae : 

 boring muscles. 



Sub-Order 4. Cardiacea. 



Dimyarian or Monomyarian. Branchial lamella much folded. Mantle closed 

 except at the two siphonal and one pedal apertures. Fam. Cardiidae : Dimyarian. 



\ 



M, 



FIG. 26. Anatomy of Cardium tuberculatum, left aspect (after Grobben, Leuckart, and 

 Nitsche, Zool. Wandtafeln). p, Foot ; go, gonad ; S, shell ; Pa, mantle ; os, labial palps ; o, mouth; 

 MI, anterior adductor muscle ; ce, oasophagus ; m, stomach ; I, digestive gland ; d, intestinal 

 canal ; grog, genital aperture ; no\, pericardial aperture of the kidney ; V, ventricle ; At, auricle ; P, 

 pericardium ; no, aperture of the kidney in the mantle cavity ; n, kidney ; Af 2 , posterior adductor ; 

 Bl, point of concrescence of the right and left ctenidia behind the foot ; a, anus ; Ak, anal chamber 

 of the mantle cavityj with anal siphon As ; Bk, branchial chamber of the same cavity with 

 branchial siphon Bs ; Br, ctenidiuui. 



(Cardium [Fig. 26].) Fam. Chamidae : Dimyarian. Valves of shell unequal. (Chama, 

 Diceras, Requienia.} To these the fossil forms Monopleuridoe, Caprinidce, Hip- 

 puritidce,<Iladiolitidce. Fam. Tridacnidae : Monomyarian. (Tridacna, Hippopus.) 



Sub-Order 5. Myacea. 



Dimyarian with folded branchial lamell?e. Tendency towards concrescence of the 

 edges of the mantle folds. Siphons very long and foot large. Fam. Psammobiidae : 

 pedal cleft of the mantle still very large (Psammobia). Fam. Mesodesmatidae, 



