112 OBSERVATIONS ON NAIADES, 



Shell triangular, ventricose, transverse, inequilateral ; posterior slope 

 wide, nearly forming a plane, and possessing numerous indistinct 

 ribs ; substance of the shell somewhat thick ; beaks somewhat promi- 

 nent and flattened ; ligament short ; epidermis yellowish, smooth and 

 shining ; rays numerous, irregular, interrupted and pointed somewhat 

 like an arrow head, on the posterior slope they are very minute ; um- 

 bonial slope carinate ; cardinal teeth large and double in both valves ; 

 lateral teeth short, nearly straight and enlarged at posterior end ; an- 

 terior cicatrices distinct ; posterior cicatrices confluent ; dorsal cicatri- 

 ces situated on the under part of the cardinal tooth ; cavity of the 

 beaks deep and rounded ; nacre pearly white. 



Remarks. — This beautiful shell has heretofore been considered as a 

 variety of U. triangularis (Barnes). It has, however, I think, char- 

 acters sufficiently distinctive to entitle it to rank among the species. 

 It differs from that species essentially in being less flat on the poste- 

 rior slope, in the umbonial slope being carinate and not rounded, 

 in being less transverse and in possessing a sharper edge along the 

 basal margin. In the triangularis the greatest transverse diameter is 

 nearer the basal margin than in this species. It is sometimes found 

 much larger than the specimen here represented, which js selected on 

 account of its perfection. My largest specimen would weigh at least 

 four times as much as this one. 



Unio Perplexus. Plate XVII. fig. 42. 



Testa ovatd, obliqud, nodulorum seriem irregulajem {fere mediis in valvulis) d 

 7iatibus ad marginem baseos currentem habente ; valvulis crassis; clivo umboniali 

 irregulariter rugato; radiis emguis et numerosis; dentibus cardinalibus modicis, 

 latcralibus longis subrectisque; margaritd ladeo-albd. 



Shell ovate, oblique, having an irregular nodulous line near the middle from the 

 beaks to the basal margin ; valves thick ; umbonial slope irregularly wririkled ; rays 

 small and numerous; cardinal teeth rather small; lateral teeth long and nearly 

 straight ; nacre milk white. 



