496 



MOLLUSCA PHYLUM VI 



hinge ivantiiKj^ and the cardinals sometimes three in one valve ; li(jament external and 

 conspicuous ; no defined area. Tertiary and Recent. 



Psammohia Lainarck {Gari Sclium.) (Fig. 819). (?) Cretaceoiis. Tertiary and 

 Recent. 



Pliorhytis Conrad ; Asaphis Modeer ; Sanguinolaria Lam. ; Tagelus Gray ; Nova- 

 culina Beiison ; Ämphichaena Phil. ; Psammosolen Risso, witli subgenus Macha Oken 

 (Fig. 820) ; Azor Leach ; and Heterodonax Mörcli. Cretaceons to Recent. 



Family 36. Donacidae Desliayes. 

 Valves equal,free, siib-trigonal, usually closed, solid ; outer surface and inner margins 

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

 2Jallial 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, and corresponding 



sockets in the opposite valve; cardinal formula t.q-,qY' ^^'^ strongest cardinal tooth often 

 bifid. Lias to Recent. 



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



sniall Segment of the resilium separate from the rest, 

 wholly internal, and in front of the beaks. 



Isodonta Buv. {Soiverbya d'Orlj.) Snb-syniniet- 

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

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

 laterals weak. Upper Eocene and Recent ; about 

 Piti. 821. lOÖ species. Subgenus Iphigenia Schuni. Recent. 



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



stage); Wiesen, near Vieiina. ^2) 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 piston or stilt within the 

 burrow ; siphons short ; hinge without lateral laminae. 



Family 37. Solenidae Leach. 



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

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

 ends, and more or less ineqidlateral, with low beahs, smooth margins, not rostrate, smooth 

 or feebly sculptured ; adductor scars narrow, 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 tioo in the left valve, with or without one or 

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

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



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

 Palaeosolen 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. 



Solen Linn. (Fig. 822), Scabbard-shaped, straight, with terminal beaks. Among 

 the numerous subgenera are : Ensis Schum. ; Pharella Gray ; Geratisolen Forbes ; 

 Siliqua Megerle ; Gultellus Schum. (Fig. 823). Tertiary and Recent. 



