408 PALAEONTOLOGY OF NEW-YORK. 



the middle of the dorsal valve. In rare instances, the plications are 

 bifurcated. Fine undulating concentric striao cover the surface, and a 

 few stronger imbricating lamellae mark the form of the shell in its 

 stages of growth. 

 The interior structure has been noticed under the generic description. 



Specimens from certain localities show no appreciable difference in the size of 

 the costse, and there is no distinct carination in the middle. 



In its young state, the shell resembles Chonetes dejlecia, but usually is propor- 

 tionally longer ; juid the absence of spines, as well as its punctate surface, are 

 distinctive features. 



The illustrations, figures 2 a, b, c, are of unusually elongate forms, where the median plica- 

 tion is not perceptibly larger than the others. 



Figures 3 a-k present the principal varieties of form and siie of the species; and figure 3 / 

 ehows a dorsal valve with bifurcating plications. 



Figure 4 is a form with more numerous and finer plications, but in other respects does not 

 show important differences. 



Figure 8 m shows the general aspect of the area and foramen, with the cardinal process. 

 The remaining figures show the interiors of ventral and dorsal valves, and the casts of the 

 same as they occur in the arenaceous beds. 



When extremely magnified, the surface presents the structure shown in figure 3 s. 



A similar or identical species has been recognized in Europe, and is published 

 under the name of Leptcena laticosta in the Bull, de la Soc. Geol. de France, 

 Tome iy, pa. 325, pi. 3 ; but it is there represented as having about thirty-two 

 plications, a greater number than any of our specimens, and it will probably 

 prove a distinct species of the same generic type. 



Geological formation and localities. In the Hamilton group in Schoharie county, 

 and extending throughout the State of New- York from near the Hudson-river to 

 Lake Erie. Some of the principal localities are in Schoharie and Otsego counties ; 

 on the shores of Cayuga, Seneca, and Canandaigua lakes, and at Darien and 

 Eighteen-mile creek. It is likewise known in rocks of the same age in Illinois and 

 Iowa. 



Tropidoleptus occidcns. 



PLATE LXII. 

 Tropidoleptut Occident : Hall in Thirteoatli Ann. Report on the State Cabinet, p. 91. 18C0. 



Shell concavo-convex, semielliptical, about as long as wide : apex of 

 the ventral valve extending beyond the hinge-line. Area linear, appa- 



