18 



GOMPARA TIVE ANA TOMY 



CHAP. 



Suh-Oider 2. Tellinacese. 

 Dimyarian with coin]>letely se[)arate siphons. Foot htrge. Gills smooth 

 Tellinidse [Tellina). Fam. Donacidse {Doiwj-), Mactridae {Mactra). 



Fam. 



Sub-Order 3. Veneracea. 

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

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

 boring muscles. 



Sub-Order 4. Cardiacea. 

 Dimyarian or IMonomyarian. Branchial lamella^ much folded. Mantle closed 

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



no. 



Fig. 26.— Anatomy of Cardium tuberculatum, left aspect (after Grobben, Leuckart, ami 



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

 Ml, anterior adductor muscle ; ce, oesophagus ; m, stomach ; I, digestive gland ; d, intestinal 

 canal ; go^, genital aperture ; moj, pericardial aperture of the kidney ; V, ventricle ; At, auricle ; P, 

 pericardium ; no, aperture of the kidney in the mantle cavity ; n, kidney ; Mn, 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 ; Bl; branchial chamber of the same cavity vfith 

 branchial siphon Bs ; Br, ctenidium. 



{Cardhcm [Fig. 26].) Fam. Chamidse : Dimyarian. Valves of shell unequal. {Chama, 

 Diceras, Eequienia.) To these the fossil forms Monopleuridce, Caprinidce, Hip- 

 puritidix,^Radiolitidce. Fam. Tridacnidse : Monomyarian. (Tridaoui, ffipjmpus.) 



Sub-Order 5. Myacea. 

 Dimyarian with folded branchial lamelhe. Tendency towards concrescence of the 

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

 pedal cleft of the mantle still very large (Psammubia). Fam. Mesodesmatidse, 



