114 
PALAEONTOLOGY OF NEW-YORK 
The casts figures 17, 18 and 19 preserve the usual characters of the casts of 
the ventral valve of S. perplana; the marks of strife, as far as seen, being more 
regular than those of the shell figure 15. These casts I infer to be similar to the 
specimen from which Mr. Conrad described Strophomena delthyris , loc. cit. 
Geological formation and localities. These specimens present the usual aspects 
of this species and variety in the Chemung group in Tioga county; near Ithaca, 
and along the line of the Corning and Blossburgh railroad. The more characte¬ 
ristic forms of those designated S. nervosa were found near Bath, and at 
Campbelltown in Steuben county. 
Strophodonta demissa. 
PLATE XIX. 
[ Described page 101 of this volume, Hamilton group.] 
The condition of this species, in the Chemung group, is illustrated in figures 
8-12. Figure 8 is a somewhat distorted cast of a ventral valve which shows no 
muscular markings. Figure 10 is a similar cast, a little more extended on the 
hinge-line; while figure 9 is extremely extended at the cardinal extremities. 
Figures 11 and 12 are views of a cast of a ventral valve scarcely differing in 
condition from the better preserve*! specimens in the Hamilton group. 
The casts and impressions of this shell in the Chemung group frequently show 
broad radiating folds, as partially illustrated in figure 9.; and the central portion 
is often produced into a nasute extension. 
Geological formation and localities. This species occurs oil Chemung creek and 
at Chemung-narrows ; also near Jefferson in Steuben (Schuyler) county, and near 
Bath; and in Cattaraugus and Chautauque counties. 
Strophodonta subdemissa. 
The species described by me as Strophomena ( Strophodonta ) subdemissa, in the 
Tenth Report on the State Cabinet, is only a variety of the S. demissa, as I sug¬ 
gested at the time. The conditions existing in the specimens then examined have 
been found in a greater or less degree, and connected with other modifications, in 
nearly all the specimens from Illinois and Iowa. 
Strophodonta plicata. 
Strophodonta plicata : Hail, Thirteenth Report on the State Cabinet, p. 90. 1860. 
This species, which was founded on a well marked specimen from rocks of the 
age of thejJIamilton group at Iowa city, has been obtained at Independence, 
Iowa, and a specimen kindly forwarded to me by Mr. Orestes St.John. 
