VII 



MOLLUSC A SYSTEMATIC REVIEW 



17 



swimming. (Pecten [Fig. 23], Chlamys). Fani. Aviculidse : Monomyarian or Hetero- 

 myarian without siphons. Valves equal or unequal (Avicula [Melcagrina], Malleus, 

 Vulsella, Perna, Inoccramus, Pinna, Melcagrina margaritifera, pearl mussel). 

 Fam. Ostreidae : Monomyarian without foot, with completely open mantle, without 

 siphons. Valves unequal, the left valve attached to the substratum. (Ostrca : 

 oyster [Fig. 24]). 



ORDER 4. Eulamellibranchia. 



The gills no longer consist of distinct filaments. On the contrary, the filaments 

 in each row and the two parts of each filament are so connected by means of 

 vascularised trabeculse or sutures as to form together a lamella or trellis -work. 

 There are, on either side, two such branchial lamellae (hence the name of Lamelli- 

 branchia), which in fact correspond with the two rows of leaflets of the typical 

 ctenidium. This order includes the majority of the Lamellibranchia. 



Sub-Order 1. Submytilacea. 



Branchial lamellae smooth. The mantle edges usually grown together only 

 between the inhalent and the exhalent apertures. Dimyarian. Fam. Carditidse : 

 with open mantle and large foot (Cardita, Vcnericardia}. Fam. Lucinidae : 

 with simple, and as a rule single, siphonal aperture. Foot often vermiform. Fam. 



no 



FIG. 25. Anatomy of Unio (Margaritana) margaritiferus, left aspect (after Leuckart and 

 Nitsche). o, Mouth ; eg, cerebral ganglion ; MI, anterior adductor muscle ; a, oasophagus ; I, 

 digestive gland (liver) ; no. nephridial aperture ; lo, aperture of the digestive gland in the stomach 

 in ; Aa, anterior aorta ; n, nephridium, the outlines given in dotted lines ; V, heart ; r, proctodfeum ; 

 Ap, posterior aorta ; M 2 , posterior adductor ; a, anus ; Vg, visceral ganglion ; Br, gill ; Bk, mantle 

 cavity ; go, gonad and ducts goi ; Pg, pedal ganglion ; p, foot. The arrows mark the direction 

 of the inhalent and exhalent streams of water. 



Erycinidse : mantle closed except at the two siphonal and the pedal apertures. 

 Foot long. (Erycina, Kellya, Lascea, Lcpton, Galeomma. ) Fam. Crassatellidae : 

 mantle open without siphons. Foot moderately developed. Fam. Cyrenidse : mantle 

 open, two siphons. Foot large. In fresh or brackish water. (Cyrcna, Oorbicula, 

 Sphcerium, Pisidium, Galatea.) Fam. Dreissensiidse (fluvial). Fam. Unionidse: fresh- 

 water ; foot large, hatchet- or wedge-shaped, two simple siphonal apertures or clefts, 

 mantle open ( Unio [Fig. 25], Painter's Mussel ; Anodonta, pond Mussel ; Mutela). 

 VOL. II C 



