104 PALEONTOLOGY OF NEW YORK. 



Many of the species originally included under this designation have been re- 

 moved by the establishment of such generic divisions as Rhynchospira, Trema- 

 TospiRA, EuMETRiA ; but, as the determination of generic values in all these 

 fossils is usually attended with difficulties, it is highly probable, and indeed cer- 

 tain that there are several distinct types of generic structure represented among 

 the commoner species referi'ed to Retzia. It will be necessary to determine 

 the value of this genus from the characters of the species Terebratula Adrieni, 

 designated by Professor King as its type ; and our observations upon it lead 

 to the belief that, strictly regarded, it presents a type of structure of rare 

 occurrence, and with present knowledge no other species can be placed in the 

 same association. 



Terebratula Adrieni was described by de Verneuil in 1845* from the lower 

 Devonian of Spain, but our fuller knowledge of the species is due to the de- 

 scriptions and figures given by Dr. CEhlert, who has identified it from the 

 lower Devonian of the Departement de la Sarthe, Sable, Brulon, etc., 

 France, f 



With the help of Dr. (Ehlert's published work and with his most consider- 

 ate personal assistance, | we have made an earnest endeavor to develop the 

 structure of the brachidium in this species. Unfortunately the specimens 

 accessible were filled with a hard, opaque calcareous material, and such details 

 as have been made out were obtained by the process of serial transverse section- 

 ing. The nature of the interior of the brachial valve and the structure of the 

 hinge-plate had already been demonstrated by (Ehlert, and the results obtained 

 in regard to the structure of the loop are so peculiar as to require corroboration, 

 which the material at hand does not permit. The following account of the 

 characters of the species represents the sum of our present knowledge of the 

 genus. 



* Bulletin de la Socifite geol. de Fiance. 2d Ser., torn. 11, p. 471, pi. xiv, figs. 10, a-d. 

 t See CEhlbrt; Etudes sur quelques Fosailes devoniens de I'Ouest de Fiance; Annales Sci. G6ol., 

 t. six. No. 1, p. 24, pi. xi, figs. 11-19. 1886. 



X Appreciating the necessity of making a thorough study of this rare species, we twice applied to Ur. 

 CEhlkrt for specimens, and he has most generously met tliese requests, not only with a number of examples 

 from the Departement de la Sarthe, but with copies of unpublished sketches of sections. This generosity 

 and spirit of helpfulness is most cordially acknowledged. 



