294 PALEONTOLOGY OF NEW YORK. 



1884. Tertbratida, White. Thii-teenlh Kept. State (ieol. liuiiaiia, p. 137, jil. xxxii, figs. 17-19. 



1884. Terelrratnla, Walcott. Monogr. U. S. Geol. Survey, vol; viii, p. 224. 



1884. Dielasma, Davidson. Bi'itish Fossil Bi'achiopoda; General Summary, p. 411. 



1887. Dielasma, de Konince. Faune du Calcaire CarboniKi-e de la Belgique, pp. 5-31, pis. i-viii. 



18S9. Terehratula, Nettelroth. Kentucky Fossil Shells, v. 155, pi. xvi, figs. 20-22. 



1890. Terehratula {Cryptonella}, Calvin. Bull. Lab. Nat. Hist. Stale Univ. Iowa, p. 174, pi. 3, iig. 4. 



1893. Didasma, Beecher and Schuchert. Proc. Biol. Soc. Washington, vol. viii, pp. 71-78, pi. x. 



Some years before the introduction of this term its distinguished author had 

 applied Phillips' name, Epithyris, to certain Permian species {Terebratulites elon- 

 gatus, Schlotheim, type) which he found to differ from Teeebratula, in the sense in 

 which the term was then current, in their " prominent dental plates and trans- 

 versely semi-elliptical moderately recurved loop." Epithyris, as used by Phillips, 

 has no significance as a generic term ; whatever value it might have was thus 

 given it by King, but the author subsequently decided to discard the term and 

 introduce a new one, Dielasma. The name has not been widely adopted, though 

 this fact appears to have come, less from any objection to, or insufficiency in, 

 the distinctive characters of the division, than to a general disposition to leave 

 all the terebratuloid shells of the Palaeozoic with the old genus, Terehratula. 

 Waagen has recognized the value of this genus and the usual facility with which 

 its species may be recognized. It is not, however, upon the characters given by 

 King that we can rely for the distinction of Dielasma from the other palaeozoic 

 terebratuloids. In external form, the convexity of both valves is generally well 

 developed, and the outline is usually elongate-oval. But in both of these 

 respects there is very considerable variation ; the development of a median 

 sinus on both valves with a plication and groove at the bottom of it, as in the 

 Terehratula turgida, Hall, of the Chester limestone, and T. vescicularis, de Koninck, 

 of the Coal Measures, produces a form at once suggestive of the typical 

 biplicate Terebratula of the Jurassic age. A general depression of the pedi- 

 cle-valve anterior to the umbo, and a corresponding elevation of the opposite 

 valve, appearing first in the Cryptonella Calvini, Hall and Whitfield, of the mid- 

 dle Devonian, is carried to an extreme in the T. bovidens, Morton, of the Coal 

 Measures. 



The apex of the pedicle-valve is closely incurved, so that in adult shells but 

 little remains of the deltidial plates. The foramen is large, quite generally 



