«70 PALEONTOLOGY OF NEW- YORK. 



C}Ttiiia Iianiiltonensis, var. recta. 



TLATE XLIV. 



Figures 36 an d37 of Plate xvii illustrate this form, which has a plane 

 flat area, without incurvation of the beak, and with angular plications. 



This form is not uncommon hi the Hamilton group, and was for a long time the 

 only one known to me in the Chennnig group ; but very recently I have received, 

 among a collection of Spirifera disjuncta from the southern part of the State, 

 u specimen which indicates that the species may have in this horizon the same 

 variations which it has in the formation below. 



Cyrtina curviliiicata [?] 



PLATE XLIV. 



Compare Cyrtia ettrtilxneaia, Whitb : Proceedings of the Boston Society of Nat. History, Vol. ix, p. 25. 



Shell rather large, obliquely subpyramidal : hinge-line equalling or 

 slightly less than the greatest width of the shell; length of dorsal 

 valve less than its width, and one-third greater than the height of 

 area of the ventral valve. Entire surface plicate. 

 Ventral valve obliquely subquadrilateral in outline, the apex turned to 

 the left : area much elevated, inclined backwards, slightly incurved. 

 The fissure has apparently been closed in the lower part, but the 

 pseudo-deltidium is broken away, and there is no evidence that the 

 upper two-thirds of the fissure has been closed at any recent period of 

 the animal's life. 

 Dorsal valve semielliptical, convex in the middle and flattened at the 



cardinal margins ; mesial fold i)rominent towards the front. 

 Surface marked by about twelve or thirteen rounded plications on either 

 side of the mesial fold and sinus. The mesial fold has four or five low 

 rounded plications near the front, while there are four corresponding 

 folds on the sinus. 



This species has the general form and proportions of the C. liamiUoriensis, and 

 occurs with that species in rocks of the age of the Hamilton group in the west. 

 It differs slightly in some of its proportions from the prevailing eastern forms of 

 that species, but not more than is observed among specimens of the same in col- 

 lections from New- York, Canada West, and the Western States. The distinsuishiii"'- 

 feature is iu the prvisence of plications on the mesial fold and sinus. The presence 



