38 PALEONTOLOGY OF NEW YORK. 



may be to some extent, although the apex of the beak is actually quite remote 

 from it. From this point to the center of the crescent extends a short furrow, 

 on either side of which lies the impression of an accessory scar, probably cor- 

 related to the urabo-lateral scars of Rhinobolus and Trimerella. Pallial sinuses 

 more or less distinct. Median septum low, but stronger than in the opposite 

 valve. 



Type, Dimholus Conradi, Hall. 



Observations. Such considerable variations in certain features occur within 

 the limitations of a single species of this genus, that we are disposed to regard 

 the recognition of the eight or nine species, which have been described, of 

 somewhat questionable authenticity. We have befoi'e us a large number of 

 finely preserved specimens of the type-species, D. Conradi, from Le Claire, Iowa; 

 Cedarburg, Wisconsin ; Hawthorne and Port Byron, Illinois ; all of which 

 indicate that variations are to be expected (1) in the development of the delti- 

 dium, which though usually quite low, may attain a very considerable eleva- 

 tion ; (2) in the size of the platform-vaults, which in both valves, are usually 

 no more than a broad, general excavation of the anterior walls of the platform, 

 narrowing abruptly into minute conical cavities, though in the pedicle-valve 

 they occasionally are long and tubular, and, in both valves, there is some- 

 times evidence of a tendency to form two or even three chambers in each vault ; 

 this may be regarded as an abnormality, but it occurs in at least six instances 

 in the examples at hand ; (3) in the position of the platform, its apex usually 

 lying at about the center of the valve, but sometimes situated more 

 posteriorly. 



Upon comparing the features of the interior of D. Conradi with the best 

 known of the other species, D. Davidsoni, Salter, we do not observe features 

 which lead to a satisfactory specific distinction. From the figures given by 

 Davidson and King,* it appears that this species is also susceptible of varia- 

 tion, for example, in the development of the median scar in the pedicle-valve, 

 which may be either a ridge or a furrow. In the description and figures of 



Lou. cU., pi. xviii, lags. U-11. 



