338 The American Geologist. J""''- ^''^■'• 
tlie top of the Upper Richmond. Since they arc not known to 
occur in the Lorraine in the area under discussion, these spec- 
ies are very useful for distinguishing hetween the Lower Rich- 
mond and the Warren beds in those parts of Indiana and Ken- 
tucky in which the DahnaneUa jugosa zone is absent. Usually 
thev are rather infrequent in the lower and middle part of the 
Dalmanella zone and do not become abundant until the upper 
part of this zone is reached. 
Sfi'oplioiiicna riigosa (Sfr. planuuibona of the Ohio Sur- 
vey) is another Richmond fossil which usually does not be- 
come abundant until the middle or upper part of the Dalman- 
ella jugosa zone is reached. The radiating striae are of medi- 
um size, and do not dilTer conspicuously in size either 
when striae on the same valve, or when strire belonging 
to opposite valves are compared. Strophomcna clongota 
James, is a variety of Stroplioinciia rugosa, diiTering only 
in the greatly extended hinge-line, which gives a subtrig- 
onal, instead of the usual subquadrate outline to the shell. 
Strophoinciia subtcuta (Str. plicata of the Ohio Survey) Is 
another variety, of larger size than the typical Sfr. planuui- 
bona ; the oblique wrinkles near the extremities of the hinge- 
line are often absent. Strophomcna clov^gata and Str. subtoita 
are typically developed in the lowest part of the Middle Rich- 
mond. Forms intermediate between these varieties and Str. 
plannmbona occur in the upper part of the Lower Richmond. 
Strophomcna ncglccta has been collected bv Dr. G. i\L Austin 
in the upper ten feet of the Lower Richmond near Clarksville, 
Ohio. It is a larger shell than Str. vctusta. often attaining a 
width of 50 mm. The convexity of the dorsal valve is much 
greater, the dei)th amounting to 1.8 mm. in shells 42 mni. long. 
The strife are much finer and closer over the larger part of the 
shell, and equal in coarseness those of the dorsal valve of 
Str. vctusta only along the anterior third of the shell. 
The chief characteristics of Stroplwnicna nutans are its 
subnasute anterior margin and the remarkable thickening of 
the interior of the ventral valve around the muscular area and 
along the anterior margin. The radiating striae are very 
fine ; some, at more or less regular intervals are slightly more 
prominent. The typical specimens are rather small, mature spec- 
imens attaining a length of four-fifths of an inch. They oc- 
