408 
PALAEONTOLOGY OF NEW-YORK. 
the middle of the dorsal valve. In rare instances, the plications are 
bifurcated. Fine undulating concentric striae 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 costae, and there is no distinct carination in the middle. 
In its young state, the shell resembles Chonetes dejlecta, but usually is propor¬ 
tionally longer; and 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 size of the species; .and figure 3 1 
shows 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 3 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 iv, 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 occidens. 
PLATE LXII. 
Tropidoleptus occidens : Hall in Thirteenth Ann. Report on the State Cabinet, p. 91. 1860. 
Shell concavo-convex, semielliptical, about as long as wide : apex of 
the ventral valve extending beyond the hinge-line. Area linear, appa- 
