262 PALEONTOLOGY OF NEW YORK. 



which terminates on their anterior face in points for the attachment of the crura; 

 on the inside of the dental sockets there is a distinct, sometimes strongly de- 

 fined ridge, extending along the lateral margin of the broad flabelliform muscular 

 area, and gradually becoming obsolete. There is also a slight median ridge 

 which becomes obsolete below the middle of the muscular area. Surface marked 

 by strong, sharply elevated radii, which alternate with finer ones, all being 

 crenulated, and the intermediate space cancellated by fine concentric striae. 



Type, Derbya regularis, Waagen. Upper Carboniferous. American example, 

 Orthisina crassa. Meek and Hayden. Upper Coal Measures. 



Observations. The validity of this genus rests entirely upon the presence 

 of the median septum in the pedicle-valve. In some other groups of the 

 brachiopods one might question the advisability of giving so much im- 

 portance to a feature of this kind, which is often very variable even in a given 

 species, but the study of the streptorhynchoid shells has led to the endorse- 

 ment of Dr. Waagen's views in regard to the value of septal characters in this 

 group. In none of the other genera, Strophomena, Orthothetes, Meekella, 

 Streptorhynchus, does such a septum exist, and so far as known, it exists only in 

 this very restricted group of fossils which first appears in the Carboniferous and 

 disappears in the Permian. In the character of the cardinal process and crural 

 plates there is no essential difference between Derbya and Orthothetes. Species 

 of the former genus are often of very great size (Z). grandis, D. regularis, Waagen, 

 D. robusta, D. Keokuk, Hall), and in consequence the cardinal process becomes of 

 very striking proportions, but structurally not different from Orthothetes, as 

 shown by all the smaller species of Derbya Of American species to be placed 

 in this genus are the Orthis Keokuk, Hall, from the Keokuk limestone of the Lower 

 Carboniferous, Orthis robusta. Hall, from the Lower Coal Measures of Illinois, 

 Hemipronites crassus,* Meek and Hayden, also from the Coal Measures, and 

 Streptorhynchus Correanus, Derby, from the Carboniferous limestone of Itaituba, 

 Province of Par;!, Brazil. In the Carboniferous of Great Britain occur the 

 species Spirifer senilis, Phillips, and Orthis cylindrica, McCoy. Dr. Waagen has 



* Meek regarded McChbsney's species, Hemipronites LasaUcnsis and H. Riclimondi, described in 1860, 

 fi'om the Coal Measures of Illinois, as synonymous with H. crassus. See Palseout. Eastern Nebraska, p. 174. 



