496 MOLLUSCA phylum vi 



Jdnge wantiiuj, and the cardiiiaU sometimes three in uiie valve; ligament external and 

 conspicuous ; no defined area. Tertiary and Eecent. 



Psammobia Laiaarck (Gari Sclium.) (Fig. 819). (?) Cretaceous. Tertiary and 

 Eecent. 



Pliorhytis Conrad ; Asaphis Modeer ; Sangninolaria Lam. ; Tayclus Gray ; Nova- 

 culina Benson ; Ampliichacna Pliil. ; Psammosolen EisKo, with subgenus Macha Oken 

 (Fig. 820) ; Azor Leacli ; and Heterodonax Morcli. Cretaceous to Eecent. 



Family 3G. Donacidae Deshayes. 



Valves equal, free, sub-triyonal, tisually closed, solid ; outer surface and inner margins 

 smooth or radially sculptured, the posterior end usually shorter and obliquely sub-trimcate ; 

 pallial sinus similar in both valves ; resilium sub-internal, sometimes amphidetic ; 

 ligament short, external, seated in a deep groove, episthodetic ; hinge-plate moderately 

 developed, usually with a posterior and anterior lateral in the right, a.nd corresponding 



sockets in the opposite valve; cardinal formula daiaii ''''^' strongest cardinal tooth often 



bifid. Lias to Eecent. 



The resiliuiu is chiefly opisthodetic and sub-internal, but some of the large species have a 



small segment of the resilium separate from the rest, 

 wholly internal, and in front of the beaks. 



Isodonta Buv. (Sowerbya d'Orb.) Suh-symniet- 

 rical, convex, laterals strong, pallial sinus deep. Jura. 

 Donax Linn. (Fig. 821). Anterior side longer, 

 laterals weak. Ujiper Eocene and Eecent ; about 

 Fii;. S21. 100 species. Subgenus Iphigenia Sclium. Eecent. 



Donax lucida Eichw. Miocene (Sarmatian Egeria Lea. Lower Eocene. 



Stage); Wiesen, near Vienna. ^^^ Hemidonax Morch (Donacicardium Vest). 



Superfamily 14. SOLENACEA Lamarck (emend.). 



Dwellers in soft sea-bottom, narrow, elongated, modified for burrowing, gaping at 

 both ends; foot elongated, distally modified to serve as a jnston or stilt toithin the 

 burrow ; siphons short ; hinge without lateral laminae. 



Family 37. Solenidae Leach. 



Shell substance as in Tellina, but the external layer shoiving its cellular structure 

 more clearly ; with a pronounced epidermis ; valves equal, free, usually truncoAe at both 

 ends, and more or less inequilateral, with low beaks, smooth margins, not rostrate, smooth 

 or feebly sculptured ; adductor scars narroiv, elongate, dorsally distributed, pedal distinct ; 

 pallial sinus small in species with anterior umbones, and vice versa ; ligament and re- 

 silium external, parivincular, seated on nymphs ; area obscure or none ; hinge-plate 

 hardly developed ; hinge often with a thickened ray crossing the valves and serving as a 

 buttress; cardinals varying from one to four in each valve, usually a single slender 

 radial laminar cardinal in the right, and. ttvo in the left valve, with or ivithout one or 

 two placed parallel with tlie hinge margin, simulating laterals; radial teeth usually 

 more or less pedunculated, rarely bifid. Devonian to Eecent. 



The Silurian forms heretofore referred to this family do not seem to belong to it, but 

 Palacosolcn Hall is scarcely distinguishable externally from some modern forms ; its hinge, 

 however, is unknown. The species of this family are mostly much modified for a special 

 mode of life, hence the variability in certain features, such as the siphons, foot and ferma- 

 ture of the mantle lobes. 



Bolen Linn. (Fig. 822). Scabbard-sliaped, straight, with terminal beaks. Among 

 the numerous subgenera are : Ensis Sclium. ; Pharella Gray ; Geratisolcn Forbes ; 

 Siliqua Megerle ; Cultellus Schuni. (Fig. 823). Tertiary and Eecent. 



