Oct. 1847.] 298 



3. Cytherea Mississippiensis. 

 Subtriangular, ventrieose, elevated, with prominent eonsentric acute ribs, 

 rather distant, and with irregular intervals and fine intermediate lines : poste- 

 rior margin somewhat curved ; basal margin profoundly rounded ; summit* 

 prominent; inner margin entire. Length iy. Height the same nearly. Rart-. 



4. Cytherea sobrina. 

 Subovate, ventrieose, polished ; with rather obtuse irregular distant, concen- 

 tric, impressed lines ; umbo entire ; extremities rounded ; base regularly 

 curved. Length 1 1-10. Height 9-10. 



Very abundant. Almost always with disunited valves. It is quite thick on 

 the anterior side towards the summit. 



5. Cytherea perbrevis. 

 Ovate-triangular, elevated, ventrieose : the posterior and anterior margins 

 equally declining and very oblique, the anterior one straight, the posterior 

 slightly curved; beaks medial; surface with numerous regular impressed lines ; 

 basal margin rounded. Length and height 6-10. 



It is of the size, and has somewhat the form of V. Astartiformis, but the 

 greater elevation, convex umbo, numerous impressed lines, and more rounded 

 base, distinguish it from that species. Rare. 



Corbis slaminea. ' 

 Suboval, convex, thin, with lamellseform concentric stride, about thirty-seven 

 in number ; posterior side with a slight fold ; beaks medial. Length 1. Rare. 



1. Lucina Mississippi ensis. 

 Orbicular, thin and fragile, with minute obsolete radiating lines ; anterior 

 side shortest, the margin obliquely truncated, angular above and elevated ; 

 inferior margins rounded ; posterior end obtusely rounded ; lunnle profound ; 

 cardinal and lateral teeth wanting. Length 9-10. Height . 



This species very much resembles L. subvexa of Claiborne, but wants the 

 impressed line on the posterior side. Rare. 



2. Lucina perlcvis. 

 Orbicular, with lamellseform concentric lines, and very minute obsolete radi- 

 ating lines, closely arranged ; beaks medial ; posterior end direct ; cardinal 

 teeth small. Length 4-10. Height rather less. Very rare. 



The shells of this subgenus are orbicular, generally punctate within, often 

 very thin and ventrieose ; cardinal teeth small and compressed, sometimes 

 obsolete or wanting ; lateral teeth none. Lucina radula, and L. edentula, among 

 recent species, belong to this group. 



1. Loripes? turgida. 



Suborbicular, very thin and fragile, with minute concentric lines ; beaks 

 medial; umbo and summit prominent ; margins regularly rounded. Height 

 6-10. Length rather more than 5-10. 



2. Loripes eburnea. 

 Suborbicular, slightly oblique, convex, with minute closely arranged concen- 



