of the Amazons Valley. 107 



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

 fossil group. 



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

 about I inch in longest measurement, and 5 lines from the 

 dorsal margirf to the anterior side. 



Anisoiliyris erecta^ Conrad, op. cit. pi. 10. fig. 16. 

 PI. V. figs. 9 a, h. 



Triangular ; both valves ventricose, not oblique ; anterior 

 end oblique, truncated; posterior side produced, cuneiform, 

 flexuous, extremity angular ; ventral margin rounded ; sum- 

 mits very prominent ; cardinal tooth comparatively small. 



Mr. Conrad had " only one specimen of this graceful sjie- 

 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 ohlique should," he thinks, '^ be omitted from the 

 generic diagnosis." 



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

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

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

 in MS. P. alius. We fail, however, to see the difference 

 between it and P. erectus^ save that P. alius is the aged or 

 mature shell, and the other the young or stunted condition. 

 The name Anisothyris (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, measuring 

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

 If inch; the rest are about 1^ inch broad, Ij inch deep, and 

 1 inch in thickness. 



Anisothyris cuneata^ Conrad, op. cit. pi. 10. fig. 12. 

 PI. V. figs. 8 a, h. 



Triangular, oblique, ventricose, solid, subequivalve ; beaks 

 terminal, summit very prominent and oblique ; anterior end 

 abrupt ; posterior end subtruncated ; 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 

 in all. 



If we venture to assert any thing positively respecting the 

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



8* 



