36 



along the middle of the shell, -^vhich may represent it, but there is not the 

 slightest indication of a division of the cavity of the umbo. The ventral 

 valve of the fossil species does not differ from that of the recent, in any 

 important character. 



It is proposed, in the work above cited; to classify the fossil species (or 

 at least such of them as have a rostral septum) as a sub-genus, Stenocisma 

 a name suggested by Mr. Conrad in 1839, in his second Ann. Rep. 

 on the Pal. N.Y. It remains, however, to be decided whether or not, 

 Bhynchonella loxia, the type of Fischer's genus, possesses a mesial septum 

 in the dorsal valve. 



The difference between the species with the rostral septum and those 

 without it seems to be the same as that between the genera Athyris and 

 Spiringera Athyris has a septum and the beak of the ventral valve usually 

 imperforate. Spirigeyia has no septum and the beak perforated. In both 

 cases the septate genus precedes the non-septate in time. 



Although I think it highly probable that a sub-division of the genus 

 will be made, I shall provisionally refer our species to Rhynchonella for 

 the pres:nt. 



< 



Rhynchonella excellens. (N. sp.) 



Fig. 17 Fig. 18. 



Fig. 17 — Rhynchonella excellent, dorsal view; 18, front riew. 



Bescr 'ptio 2. — Sub-circular or obscurely sub-pentagonal ; slope on each 

 aide of the beak concave ; sides broadly and uniformly rounded ; front 

 margin, with about one-third the width in the middle, either straight or 

 gently concave. Greatest width about the mid-length or a little in front 

 thereof; width about one-sixth or one-fifth greater than the length. 

 Ventral valve moderately convex ; most elevated at about one-third the 

 length from the beak ; mesial sinus, at the front margin, about one-third 

 the whole width of the shell; flat in the middle or uniformly concave? 

 deeply indenting the edge of the dorsal valve, (fig. 18) but becoming 

 nearly obsolete at about one-fourth the length from the margin, and dis- 

 appearing entirely at about half the length of t'le value. Umbo 



