52 THE NAUTILUS. 



72 mm. The specimen found b^' Dr. Cooke on July 7tli hud a 

 length of 90 mm. Their occurrence thus in a colony and of 

 different ages estahlishcs cohongoronta as a meml)er of the naiad 

 fauna of that vicinity. That the locality is well fitted for them 

 is shown by the great beauty of the specimens, which are highly 

 polished, much rayed with dark green on a ground color vary- 

 ing from yellowish-olive to light green and to very dark 

 brownish-olive. The shells are perfect except for a slight 

 erosion of the beaks. The posterior ridge is high and fairly 

 sharp. The angle at the junction of the posterior and ventral 

 margins is prominent. 



Other naiads found at the same station were Lampsib's cari- 

 nsa (0), Strophitus edentalus undtdatus (1), Symphynota viridis 

 (1 dead), Alasmidonta. undulata (2), Mnrgaritana marginaki (6), 

 Unio complanntus (m;iny), Unio prodactus (many). 



Until the present time Unio productus and Unio fisherianns 

 have been the only naiads in this vicinity which have offered 

 any great difficulty in identification. They are not yet well 

 understood. Further study may show that they belong to the 

 same species or, on the other hand, it may bring to light char- 

 acters which will more surely differentiate them. Lampsilis 

 cariosa and L. ochracea have been confusing to some students. 

 Simpson ' has pointed out the differences between them. To 

 me very rarely have they offered any difficulty. The coming 

 of cohongoronta into this neighborhood may probably lead to 

 problems m future times. With passing time this shell, in ac- 

 commodating itself to new surroundings, may be modified in 

 form and color, and these modifications may trend in the direc- 

 tion of cariosa, making it difficult to distinguish between the 

 two species. Whence cariosa came and how and why it came 

 we have no positive knowledge, but it has been here since pre- 

 historic times and its make-up must be well established and 

 best suited to its environment. We hardly may expect any 

 great change in this species. Cohongoronta is a new arrival. 

 It may find conditions here approximately like those in its an- 



1 Nautilus, VIII, pp. 121-123, 1895. Both species are figured, but the 

 legends beneath the figures should be transposed. 



