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 (D. 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 Pard, Brazil. In the Carboniferous of Great Britain occur the 
species Spirifer senilis, Phillips, and Orthis cylindrica, McCoy. Dr. Waagen has 
* Meek regarded McChesney’s species, Hemipronites Lasallensis and H. Richmondi, described in 1860, 
from the Coal Measures of Illinois., as synonymous with H. crassus. See Palseont. Eastern Nebraska, p. 174. 
