296 
PALEONTOLOGY OF NEW YORK. 
total difference in the expression of the two. The lateral divisions of the plate 
in Cryptonella have become merged with the valve and lost in Dielasma. 
The median division, which is also to a certain extent myiferous in Crypto¬ 
nella, is carried to an extreme of development in Dielasma, where it forms a 
distinct platform. In Dielasma the crura are greatly abbreviated. The 
descending lamellm of the brachidium are attached to, and are continuous with 
the crural plates, as far as the latter extend. The crural apophyses on the 
upper margins of these lamellm are developed behind the points where the 
lower margins of the lamellae are free from the crural plates. The lateral parts 
of the bra'chidium are more or less divergent, the recurvature of the ascending 
lamellae rather short and the entire structure does not extend beyond the mid¬ 
dle of the shell. The ascending lamellae are very fragile and usually destroyed 
in fossilization. 
It is thus evident that the differentials of Dielasma are highly developed and 
these having become fixed at the opening of the Carboniferous period, species 
of the genus abounded until the close of the Permian. 
In American faunas the specific values of these forms have not been thor¬ 
oughly determined, but we may quote as characteristic examples of Dielasma, 
the following: Terebratulaformosa and T. turgida, Hall, of the Warsaw lime¬ 
stone, T. Rowleyi, Worthen, and T. Burlingtonensis, White, of the Burlington 
limestone, and T. hovidens, Morton, of the Coal Measures. The type of structure 
was, however, well defined in the Devonian, and the Cryptonella Calvini, Hall 
and Whitfield, of the middle Devonian of Iowa, is an excellent representative 
of the earliest forms of the genus. The great specific representation of the 
genus in the later Carboniferous faunas has been demonstrated by the labors 
of De Koninck^ and Waagen.I 
It has been suggested by Waagen:]: that the Terebratula Lincklmi, Hall, of 
the Hamilton fauna of New York, might prove to be an early representative of 
Dielasma. Reasons have already been advanced to show that this species, with 
*Faune ciii Calcaire Carbonifei'e de la Belgique; Ann. du Mas. N. Y. d’Hist. Nat. de Belg., vol. xiv, 
pt. vi, pp. 5-31, pis. i-viii. 1887. 
t Palaeontologia Indica; Productus-limestone Fossils, pp. 336-359, pis. xxv-xxvii. 1882. 
J Op. cit., p. 337. 
