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 lamellae 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 lamellsB are developed behind the points where the 

 lower margins of the lamellae are free from the crural plates. The lateral parts 

 of the brachidium 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: Terebratula formosa and T. turgida, Hall, of the Warsaw lime- 

 stone, T. Rowleyi, Worthen, and T. Burlingtonensis, White, of the Burlington 

 limestone, and T. bovidens, 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.f 



It has been suggested by WaaqenJ that the Terebratula Lincklani, Hall, of 

 the Hamilton fauna of New York, might prove to be an early representative of 

 D1ELA8MA. Reasons have already been advanced to show that this species, with 



* Paune du Calcaire Carboniftre de la Belgique ; Ann. du Mus N. T. d'Hist. Nat. de Belg., voL xiv, 

 pt. vi, pp. 5-31, pis. i-viii. 1887. 



t Palaontologia Indica ; Productus-Iimestone Fossils, pp. 336-359, pis. xxv-xxvii. 1882. r 



Xpp. cU., p. 337. 



