186 
PALEONTOLOGY OF NEW YORK. 
adductor impression central, elongate and very small. The brachial valve 
bears a median septum which extends for one-half the length of the shell; 
being divided toward its posterior extremity, each branch supporting one 
process of the divided hinge-plate. The crura are long, slightly curved and 
somewhat expanded at their tips; between these there is a small, simple, 
cardinal process. 
External surface covered with exceedingly fine, filiform, papillose, concentric 
lines. Shell-substance fibrous, impunctate. 
Type, Terebratula cuneata, Dalman. Wenlock and Niagara groups. 
Observations. The elongate-triangular outline and the surface-ornamenta¬ 
tion of the only species that can now be referred to this group afford external 
peculiarities which at present appear to be of considerable importance. Many 
rhynchonelloids show slight evidence of a reversal of the relative convexity of 
the two valves in the process of growth; here, however, this reversal is a 
feature which is conspicuously apparent in the mature shell, on account of its 
accomplishment only at a late stage in its development; hence the umbonal 
region of the sinus-bearing valve is convex, and that of the fold-bearing valve 
concave, for nearly one-half the length of the shell.* 
The great size and elevation of the deltidial plates and their unusual con¬ 
vexity are also peculiarities which characterize the mature condition of the 
shell. In the interior the brachial valve has somewhat the same construction 
as in Rhynchotrema, that is, shows coexistence of a median septum, slender 
and simple cardinal process and discrete hinge-processes. Their relative de¬ 
gree of development, however, is different, and in the structure of the pedicle- 
valve, its dental lamellae and muscular scars, the diversity is notable. We are 
inclined to regard these differential characters as forming a good basis for the 
generic (or subgeneric) distinction of the species, f The American representa- 
* For a full discussion and illustration of the nature of this reversal in contour and also of the develop¬ 
ment of the deltidial plates, see Memoirs New Yoi-k State Museum, vol. i, No. 1, xip. 47-51, xil. iv, figs. 12- 
22. 1889. 
t It is necessary to cori’ect here certain errors in the original descrij)tion of the genus Rhynchotketa. 
One is an error of assunijition, that the crura united to form a terebratuloid loop ; subsequent examinations 
jirove them discrete, somewhat curved and explanate at their extremities. Another, that the substance of 
the shell is finely punctate. 
