24© 
PALAEONTOLOGY OF NEW-YORK. 
identity of our species with those given by Sowerby 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 xlii : 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 lamellae are extremely extended 
towards the front of the shell, and fig. 15 is somewhat similar. This extreme cha¬ 
racter has been observed more frequently in the small gibbous forms, but it like¬ 
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 Plate xli, and by figure 19 of 
Plate xlii, is that described by Mr. Conrad as Delthyris perlatm*, and is cited 
by Mr. VanuxemI as Delthyris prolata. The Delthyris chemungensis\ of Conrad 
is thus described : 
“ Delthyris chemungensis. Triangular, ventricose, with numerous slender ribs; 
upper valve with the mesial fold wide, 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. Locality, 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 Spirifer a 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. 
f Report on the Third Geological District of New-York, pp. 179, 185. 
t Journal of the Academy of Natural Sciences of Philadelphia, Vol. viii, p.263. 1842. 
