5l6 TRANSACTIONS OF THE WAGNER FREE 



This group comprises the prototypes of numerous forms which subse- 

 quently developed distinctness. Many forms are described under the name of 

 Ctenodoiita which belong in the Nuculidce or Ledidce. There is no harmony 

 between the Manuals but I follow Zittel for Ctenodonta and Fischer in the case 

 of Cucidlella, disregarding synonj'my. It is possible that with further re- 

 searches the intermediate links may play havoc with this family, but at present 

 it seems convenient. 



Family NUCULID/E. 



Shell compact, closed, with the teeth in two series meeting below the 

 umbones, separated by a chondrophore, area represented by an obscure lunule 

 and escutcheon; no ligament, but a wholly internal amphidetic, alivincular 

 resilium ; internal layer of shell nacreous ; mantle lobes free, without siphons ; 

 ventricle sub-duplex, dorsal to the rectum, with anterior and posterior aortas; 

 dioecious. 



Lower Silurian to recent fauna. 



Ex. Nucula, Acila. 



Family LEDIDtE. 



Like NucididiF, but with the ligament variable, the resilium sometimes ex- 

 ternal, or absent, the iaternal layer subnacreous or porcellanous, the ends of 

 the shell partly gaping; the mantle lobes more or less united ; with complete, 

 sometimes elongate, siphons; the pallial line usually sinuated ; the ventricle 

 embracing the rectum, with anterior and posterior aortas. 



Silurian to the recent fauna. 

 Ex. Leda, Yoldia, Neilonella, Mallelia, Tindaria, Neilo, Sarepta, etc. 



In the (Tertiary-recent) genus Malletia, the ligament is external, opis- 

 thodetic and parivincular, but the archaic features of the soft parts are retained. 

 Sarepta, a very minute recent form, has a simple pallial line, but the other 

 characters retain it in this family. A survey of the genera of Ledidce shows 

 gradations leading to Ctenodontidce, Nuculidce and Arcidce. 



Superfamily ARCACEA. 



Shell of varied form, usually with a pilose epidermis, porcellanous, with 

 tubuliferous non-prismatic external layer; taxodont; area typically amphi- 

 detic ; ligament external, ali- or multivincular ; gills filibranchiate, with the fila- 

 ments usually reflected ; pleural ganglion fused with the cerebral ; mantle 

 lobes free, without siphons, the pallial line simple; otocysts impervious; the 

 ventricle more or less doubled, and laterally separated, with both aortas pres- 

 ent ; foot variable, deeply grooved ; byssogenous ; marine or fluviatile. 



Family MACRODONTID.E. 

 Shell arciform, with the posterior hinge-teeth elongated, tending to be 

 parallel to the hinge margin ; ligament multivincular. 



