18 COMPARATIVE ANATOMY CHAP. 



Sub-Order 2. Tellinacese. 



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

 Tellinidse ( Tellina). Fam. Donacidse (Donax), Mactridse (Mactra). 



Sub-Order 3. Veneracea. 



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

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

 boring muscles. 



Sub-Order 4. Cardiacea. 



Dimyarian or Monomyarian. Branchial lamelke much folded. Mantle closed 

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



710, 



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, oesophagus ; m, stomach ; I, digestive gland ; d, intestinal 

 canal ; go 2 , genital aperture ; noi, pericardial aperture of the kidney ; V, ventricle ; At, auricle ; P, 

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

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

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

 branchial siphon Bs ; Br, ctenidium. 



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

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

 Eadiolitidce. Fam. Tridacnidse : Monomyarian. (Tridacna, Hippopus.) 



Sub-Order 5. Myacea. 



Dimyarian with folded branchial lamellae. 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. Mesodesmatidse, 





