INVERTEBRATE PALEONTOLOGY. 157 



CYRENIDiE. 

 Genus CYRENA, Lamarck. 



Synon.— Cyrena, Lam. (1318), Hist.. V, 561, Ma : and 2d ed., VI. 271— Schweigg. (1820), Natgsch., 707 ; 

 (as a subgenus of CtycJos).— Fferuss. (1821), Tab. Syst., xliii.— Blainv. (1824), Diet. Sci. 

 Nat., XXXII, 336 ; and (1825), Mala,.. 552, (as a subgenus of Cyclae).— Bronu (1838), 

 Leth., 598. — Hinds (1844),Voy. Sulpb. Zo6l.,6(i ; (asa distinct gonns). — H.and A. Adams 

 (1857), Genera Recent Moll., II. 545, (as a distinct, genus).— Prime (I860), Synon. 

 l'am. Cyclades, Proceed. Acad. Nat. Sci. Philad. ; and (1864), Ann. Lye. Xat. Hist. 

 New York, VIII, 83 ; also (1865), Smithsonian Miscell. Coll., 1 1, (as a distinct genus). 



Cyanocyclas, Feruss, | 1818), Diet. Sci. Xat.. XII. 280. » 



Polymesoda, Rafiuesqne (1820), Monogr. Bivalves (as subgenus Cyclae). 



Gelonia, Gray (1842), Synon. IJrit. Mus., 79 and 91. 



Fiji tu, II. and A. Adams ( L858), Genera Receut Moll., II, 651 (as subgenus Cyrena i. 



Elym. — Kvpyvn, the name of a nympb. 

 Exam]}- — Cyrena Ceylanica, Chemnitz (sp.). 



Shell generally thick, suborbicular, or more or less transversely oval, 

 usually gibbous ; surface concentrically striated ; beaks often eroded; hinge 

 with three cardinal teeth in each valve, the anterior one of the right valve 

 and the posterior of the lefl being smaller, and the others bifid; lateral teeth 

 remote, smooth, more or less compressed, but not much elongated ; pallial 

 line with a small sinus, or nearly or quite simple * 



H. and A. Adams propose to divide this genus into two sections, or sub- 

 genera, as follows : 



1. cyrena, Lamarck (typical). 



Shell solid, subcordate, gibbous, not much produced posteriorly. — 

 (Type as above.) 



2. egeta, H. and A. Adamsf {Anomala, Desh.; not Koppe). 



Shell thin, and more or less ventricose ; anterior side short; 



posterior produced and subrostrated. — C. angu/afa, Desh. 



From Corbicula, with which palaeontologists often confound this genus, 



it may be at once distinguished by its shorter, generally thicker, and always 



unstriate lateral teeth. It is more nearly allied in the characters of its hinge, 



' Mr. Prime lias called attention to the fact that, in the existing American species of this genus 

 and Corbicula, the pallial line is always distinctly sinuous, the sinus being sometimes comparatively 

 deep and sharply angular ; while in foreign species it is lint slightly, or sometimes not at all, sinuous, 

 S:. far as I have had an opportunity to determine, all of our far-western fossil species, excepting C. Dako- 

 tensis, have a more or less distinctly sinuous pallial line; hut in no instance have I seen the sinus so 

 deep and sharply angular in the latter, as represented by Mr. Prime in some of the recent species. 



tit will be observed that this section hears very nearly the same relations to the typical forms of 

 tin- genus that the fossil group Lcpteslhes bears to the typical section of Corbicula. Indeed, Cyrena i Eyt ta ) 

 Floridana, Conrad, resembles quite closely, in form and external appearances, Corbicula (Lepteslhts) 

 fracta, M 



