114 PALEONTOLOGY OF NEW-YORK 



The casts figures 17, 18 and 19 preserve the usual characters of the casts of 

 the voutral valve of S. perplana; the marks of strire, as far as seen, being more 

 regular than those of the shell figure 15. These casts I infer to bo similar to the 

 apooimcn 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 deniissa. 



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 whicli 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 preserved 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 nasnte extension. 



Geological formation and localities. This species occurs on Chemung creek and 

 at Chemung-narrows ; also near Jefferson in Steuben (Schuyler) county, and near 

 Bath ; and in Cattaraugus and Chautauque counties. 



Strophodonta suhdemissa. 



The species described by me as Strophomena {Strophodonta) subdemifsa, 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 : Hall, 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 the Hamilton group at Iowa city, has been obtained at Indcpcndonco, 

 Iowa, and a specimen kindly forwarded to mo by Mr. Orestes St.John. 



