146 BUI.LET1N 31 146 



low. It is moreover less triangular than t3^pical zc'inue/iszs. In 

 the Conradian collection at the Phila. Academy there are two 

 irigoniata-lxkQ specimens, very thick, wdth peripheral rugosities, 

 which are probably the ones figured in Harris' Reprint of Con- 

 rad's work, pi. 20, fig. 6 and labelled "sndrrassa Lea". These 

 appear to be the true "discoida//s'' of Conrad. 



T5'pical trigoniata is not very common in the Claiborne 

 sand, but in certain Jacksonian beds in Arkansas is rather abund- 

 ant, generally the onh^ member of the genus present. 



Meretrix trigoniata Lea, PI. 47, Figs. 1-3. 



Cytherea trigoniata Lea, Cont. to Geol., 1833, p. 67, pi. 2, fig. 44. 

 Dione discoidalis Con., Am. Jr. Conch., vol. i, 1865, p. 6 (not C. dis- 



coidalis Con. Foss. Sh. &c. ) 

 Cytherea trigoniata de Greg., Mon. Faun. Eoc. Ala., 1890, p. 218, pi. 



34, figs. 15-22. 

 Lea's original description. — Shell somewhat inflated, triangular, 

 concentrically and minutely striate, substance of the shell rather thick ; 

 beaks moderately elevated and recurved ; lunule long, elliptical ; teeth 

 moderately large, excavation of the palleal impression deep and rounded ; 

 cavity of the shell deep, subangular ; margin entire. 



Diam. .6, Length .9, Breadth i.i, of an inch. 



In outline the trigoniata resembles closely the siibcrassa. It is, how- 

 ever, less thick in the substance of the shell, has much finer striae, and is 

 entirely without crenulations on the margin. 



We strongly suspect that Conrad's name discoidalis has be- 

 come attached to this species because of his and others' state- 

 ments making the tw^o synonymous. Lea's figure is what has 

 been at the bottom of the determination of the species. 



Conrad's description of discoidalis does not tall)' well with 

 this form. (See Foss. Shells Tert. Form., Aug. 1833. p. 37). 

 Such expressions as "suborbicular" , "Inner margin crenulated; 

 lunule cordate, not very distinct. Diameter i inch." and the 

 name itself, disc-like : seem to suggest quite a different form. In 

 fact fig. 6 of the proof plate 20, incorporated by Harris in his re- 

 print of Conrad's Fossil Shells seems to represent Conrad's dis- 

 coidalis. Figure 2 styled discoidalis should accordingly be la- 

 belled trigoniata Lea. 



