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 York State Museum, vol. i, No. 1, pp. 47-51, pi. iv, figs. 12- 

 22. 1889. 



t It is necessary to correct here certain errors in the original description of the genus Rhtnchotrbta. 

 One is an ei-ror of assumption, that the crura united to form a terebratuloid loop ; subsequent examinations 

 prove them discrete, somewhat curved and explanate at their extremities. Another, that the substance of 

 the shell is finely punctate. 



