946 PALEONTOLOGY OF NEW-YORK. 



identity of our species with those given by Sowerbt and Phillips under 

 different designations, and have adopted the synonymy given by David- 

 son, De Koninck, De Verneuil, Sowerby and others, under this species. 

 It has a wide geographical extent and great vertical range in the Che- 

 mung group, and presents a variety of form and proportions which it 

 would be quite impossible fully to illustrate. The more important phases 

 I have endeavored to show in the figures on Plates xli and xlu : the 

 greater number of these are of casts. 



The predominating characters of the casts of ventral valves are shown in 1 - 4 

 and 7, 8, 10, 13, 19 & 20 of Plate xli. 



In figure 5 we have a form where the dental lamella; are extremely extended 

 towards the front of the shell, and fig. 15 is somewhat simihir. This extreme cha- 

 racter has been observed more frequently in the small gibbous forms, but it Hke- 

 wise occurs in the large and less convex ones. The extremely extended forms with 

 mucronate extremities are less common than the others; but in some localities, 

 nearly all the individuals found have that character. 



The form represented by figures 19 and 20 of Phite xli, and by figure 19 of 

 Plate XLii, is that described by Mr. C!onrad as Delthyns perlatiis*, and is cited 

 by Mr. VANUXEMf as Delthyns prolata. The Delthyris chemunffenstsX of Conrad 

 is thus described : 



"Delthykis chemungensis. Triangular, ventricose, with numerous slender ribs; 

 upper valve with the mesial fold Avide, convex or rounded and ribbed like the 

 sides, except that the ribs bifurcate, about thirteen in number; area of inferior 

 valve very wide; mesial fold profound. Length, one inch; width, one inch and 

 a half LocaHty, Chemung-narrows, New- York : in Devonian shale." 



There is no other Spirifer known to me in the locality cited, or in the Chemung 

 group, which will correspond with the description, except the 8pinfera disjuncta. 



One of the remarkable features in the dorsal valve is the duplication of the 

 mesial fold, or the marked longitudinal furrow along the centre. This feature can 

 be observed in all gradations; and while in some localities there is scarcely a 

 specimen to be found having this character, in other places it prevails in the larger 

 number of individuals. This depression is usually narrow and sharp, but in some 

 individuals it is broader, as shown in figures 13, 15, 16, Plate xlii. The fold is 



• Annual Report on the Palaeontology of New- York, p. 54. 1841. 



t Report on the Third Geological District of New- York, pp. 179, 185. 



X Journal of the Academy of Natural Sciences of Philadelphia, Vol. viii, p. 263. 1842. 



