130 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



black colours, commonly described as specific characters. Not 

 improbably this is a protective device on the part of the mollusc ; 

 the writer's experience would indicate that such is the case. 



Family SPH^RIID^, Bourguignat, 1885. 



Shell small, quadrate, ovate, or trigonal, inflated, concentrically 

 striate, umbones sub-central or somewhat anterior, hinge more or less 

 developed, cardinal teeth small and feeble, laterals short and weak, 

 ligament deep-seated and inconspicuous, pallial line simple; animal 

 hermaphroditic, viviparous; habitat fluviatile and lacustrine. 



Sp/ia'n'u>/i and Musculiiim represent this family in the Nearctic 

 Region. The species are characteristically widespread; none of the 

 recent Californian forms are confined west of the Rocky Mountains, 

 and at least three are common to the Pala^arctic Region. 



Genus Sph^erium, Scopoli. 



Tellina (sp.), Linne, 1758 {T. cornea, L.) ; Sphcerium, Scopoli, 1777 

 {T. cornea, L.) ; Cyclas (sp.), Bruguiere, 1798, Lamarck, 1799 

 {T. cornea, L.), not as restricted by Link, 1807, and Dall, 1903 ; 

 Cornea, Megerle, 1811 (C communis, Meg. = T. cornea, L.) ; 

 Cycladea, Rafinesque, 1815 (emended form of Cyclas); Cycladites, 

 Kriiger, 1848 ; Corneola, Clessin in Westerlund, 1890 {T. cornea, 

 L.), not Corneola, Held, 1837. 

 Type, Tellina cornea, Linne. 



Sub-genus Amesoda (Rafinesque). 



Amesoda, Rafinesque, 1820 {Cyclas similis, Say) ; P/s«<m, Bourguignat, 

 1857 {Cyclas rivicola, Leach), not Pisum, Megerle, 1811; 

 SphcBriastrum, Bourguignat, 1854 {Cyclas rivicola. Leach); 

 Cyrenastrum, Bourguignat, 1854 {Cyclas solida, Normand). 



Type, Cyclas similis, Say. 



^Mh-^GWwn Sphcerium, s.s. Shell of small size, averaging 10 mm. in 

 length, sub-solid, quadrate-discoidal, finely striate, beaks low, hinge- 

 teeth small, poorly developed, cardinals oblique to hinge, laterals 

 rather short ; habitat lacustrine and fluviatile. 



Sub-genus Amesoda. Shell similar to preceding, but usually larger 

 (averaging 15 mm. in length), and more solidly built, trigonal or 

 quadrate-trigonal in outline, completely coarsely striate, beaks 

 elevated, hinge heavier and better developed, one of the cardinals 

 commonly bifid; Sphcerium stage passed during eai'ly adolescence; 

 habitat similar to Sphmrium. 



Spnj.KiuM coRNECM (Linue). 

 Tellina cornea, Linne, 1758; Cyclas rhomboidea. Say, 1822. 



Shell of moderate size, quadrate or quadrate-elliptical, moderately 

 or strongly inflated ; habitat lakes and quiet streams. 



Jioi'cal portions of Palajarctic and Nearctic Regions. Yukon, Fraser, 

 and Columbia Systems. 



Quaternarv : Loess of eastern States. 



