BRACHIOPODA. 297 



T. simulator. Hall, and some others, possessing a narrow and slightly recurved 

 but decidedly elongate brachidium, conveniently constitute a subdivision of 

 Crtptonella. But there are other Devonian species in which the loop is far 

 more like that of Dielasma, as for example the Terebratula Romingert, Hall, a 

 form widely distributed in the Hamilton fauna of North America ; and the 

 T. {Cryptonella) lowensis, Calvin, a large, biconvex, and often beautifully pre- 

 served shell, from the middle Devonian of Fayette and elsewhere in Iowa* In 

 this shell the hinge-plate is constructed as in Cryptonella and is not adherent to 

 the bottom of the valve as in Dielasma, though it may be close to it ; the crura 

 also arise normally from the lateral 

 divisions of this plate. With these 

 distinctive differences from Dielasma, 

 the resemblance to the latter genus in 

 the form of the brachidium is striking, 



its descending branches being highly fig. 215. Ta-ebratulaiCranana) Komingeri, H&U. 



,.  .1 T 1 I 1 An oatline showing the Dielasma form of the brachidium 



divergent, the ascendmg branches ab- and the divide.i hinge-piate. 



ruptly recurved, making a broad, gentle curvature above ; at the same time this 

 recurved band is so very fragile as to be almost invariably destroyed. The entire 

 length of the loop, as in Dielasma, and in contradistinction to Cryptonella and 

 EuNELLA, is aVjout one-third that of the brachial valve. This peculiar structural 

 variation may be designated by the term Cran^na.! Probably other American 

 Devonian species, besides the two mentioned, will be found to belong to this 

 group when satisfactory evidence of their internal structure has been obtained. 

 Recent observations by Beecher and SchuchertJ upon the development of 

 the brachidium in Dielasma turgida, Hall, of the St. Louis limestone, show that in 

 its earliest observed condition, in a shell about 4 mm. in length, it is altogether 

 like that of primitive forms of RENSSELiERiA {R. mutabilis) ; the lateral branches 

 uniting by simple coalescence to form a triangular median plate, which is not 

 thickened along the line of suture, either below, as in R. mutabilis, or above, as 



* Caltuc, Bull. Lab. State Univereity of Iowa, p. 174, pi. iii, fig. 4. 



t To Miss AoirBS Cbaitb, of Brighton, England, an associate in the later laboi's. of Dr. Thomas David- 

 sou, and an astute student of the Brachiopoda. 



X Development of the Brachial Supports in Dielasma and Zygospira, op. cit. 1893. 



