266 PALEONTOLOGY OF NEW YORK. 



The latter genus was established in the same year (1859) as Renssel^ria, 

 and was founded upon a small plano-convex species with smooth exterior, the 

 Rhynchondla glans-fagea. Hall, from the Upper Helderberg group. 



The original description and figures illustrative of this genus did not give the 

 structure of the brachidium with accuracy, but this was afterwards made out 

 by Dr. Carl Rominger and shown in his excellent figure, given in the Sixteenth 

 Report on the New York State Cabinet of Natural History (p. 47, 1863), and 

 which is reproduced in this place. Subsequent examinations have endorsed the 

 correctness of Rominger's determination and also show that the type upon which 

 it is constructed is the same as in Renssel^ria ; the actual differences of 

 expression appearing (1) in the shortness of the jug.al processes, (2) in the less 

 extended anterior plate, and (3) in the prominent median crest or ridge upon 

 that plate, not produced posteriorly into a rod-like apophysis. 



It has already been observed that in the early (Lower Helderberg) species of 

 Renssel.bria, such as R. mutabilis, some of these features, as the length of the 

 crura, and the development of the median process are not so advanced as in 

 the later and typical species of the genus, nor as in R. (Beachia) Suessana. 



Upon farther examination of more extensive material it is found that this 

 type of brachidium-structure has, without essential variation in itself, been 

 associated in Devonian faunas with a considerable variety of external form and 

 ornamentation of the shell. This fact has already been observed by some 

 authors. Reference has been made to (Ehlert's observation* of a similar 

 brachidium, with a perforated hinge-plate, in certain small plano-convex or 

 biconvex plicated or ribbed species, Terebratula Guerangeri, Centronella Bergeroni 

 and C. Gaicdryi, from the Devonian of western France, all of which he has 

 referred to Centronella on the basis of this structure. Derby has found a 

 brachidium of similar structure, though somewhat more nearly approaching 

 that of REN88EL.ffiRiA, in another small and strongly ribbed biconvex species 

 from the Devonian of the State of Matto Grosso, Brazil, described by him 

 under the name Centronella ? Margarida.f 



♦BulL <ie la Soc. d'Etudes Scientif. d'Anjfers, 1883, pp. 1-11, pL i, figs. 1-11; pi. ii, dge. 1-6; ditto, 

 1885, pp. 1-5, plate, figs. 1-19. 



t Nof4 sobre a Qeologia e Paleontologia ile Matto Grosso : Archives do Museu Nacional do Rio de Jan- 

 eiro, voL iv, p. 84, figures. 1890. 



