of the Amazons Valley. 107 



are reversed, as in all the other species of this remarkable 

 lossil group. 



The specimens are very uniform in size, being, wlien adult 

 about 4 inch m longest measurement, and 5 lines from the 

 dorsal margm to the anterior side. 



Amsothyris erecfa, Conrad, op. cif. pi. 10 fio- 16 

 PI V. figs. 9 a, 5. • ^- • 



Triangular; both valves ventricose, not oblique; anterior 

 encl.obhque, truncated; posterior side produced, cuneiform, 

 tlexuous, extremity angular; ventral margin rounded; sum- 

 mits very prominent ; cardinal tooth comparatively small. 

 _ Mr Conrad had " only one specimen of this graceful spe- 

 cies the largest of the genus known." " The valves are 

 much less unequal than in the preceding species, and the erect 

 beaks give it a very different contour from the others. The 

 character oblique should," he thinks, "be omitted from the 

 generic diagnosis." 



Mr. Conrad makes a note at the end of the separate copy 

 ot his paper (obligingly lent me by Mr. Bates), with an out- 

 ime of what he evidently considers a new form and has named 

 m Mb. J^. alius. We fail, however, to see the difference 

 between it and P. erectics, save that P. altus is the aged or 

 mature shell and the other the young or stunted condition, 

 i He name AmsotJiyris {Pachydon) erecta must be retained 

 and include both. 



This is a somewhat rare form : there are eighteen speci- 

 mens two only of which attain a considerable size, measm-ino- 

 2 inches broad and If inch deep, and having a thickness of 

 I4 inch; the rest are about 1^ inch broad, H inch deep, and 

 1 inch m thickness. 



Amsothyris cuneata, Conrad, op. cif. pi. 10 fio- 12 

 PI. v. figs. 8 a, J. 



Triangular, oblique, venti-icose, solid, subequivalve ; beaks 

 terminal, summit very prominent and oblique ; anterior end 

 abriipt ; posterior end subtnincated ; disk somewhat flattened 

 mesially; umbonal slope rounded, undefined, nearly mar- 

 ginal ; ventral margin nearly straight posteriorly ; cardinal 

 tooth oblique. 



This is a well-marked species and readily separated. We 

 have about fifty specimens before us, and, save in the differ- 

 ence due to age, the characteristic trigonal form is maintained 



m n 



If we venture to assert any thing positively respecting the 

 nature of the habitat of these shells in their living state, it seems 



