BRACHIOPODA. 
291 
marked, and limited by a prominent border. Dorsal valve with a narrow 
hinge-area transversely or longitudinally striate or both, and marked in the 
center by a deltidial scar. Cardinal process double, each division notched or 
bidentate at the extremity; muscular area quadrangular, occupying a more or 
less elevated callosity, and a central carina rising from the lower part of this 
area is sometimes produced into a spiniform process in the center of the 
cavity.”— Hall, Twenty-eighth Rept. N. Y. State Mus. Nat. Hist., pp. 153,154. 
Type, Strophodonta semifasciata , Hall.* Niagara group. 
Observations. The shells included in this genus are essentially Stropheo- 
dontas with the relative convexity of the valves reversed. It is evident from 
a careful inspection of any well-preserved example of Strophonella, as already 
shown from a study of very young individuals of S. striata, Hall, that in early 
stages of growth the convexity of the valves is normal, reversion of convexity 
obtaining with growth, f 
* Since this is the first-named species in the list it will probably be regarded by students as the “ type ” 
of the genus, while in fact the generic description had reference to the better known Lower Helderberg 
forms, and this name was written first in a list of species given in their geological sequence, and including 
in addition to 8 . semifasciata, 8. Leavenworthana, S. cavumbona and 8. punctulifera (the last two probably 
identical), of the Lower Helderberg group, 8. ampla, of the Upper Helderberg group, 8. reversa, and 
8. ccelata, of the Chemung group; the interior characters of 8. semifasciata were less known than in any of 
the others, and it was included in the group from its resupinate form and the character of the cardinal area, 
deltidium, etc. In 8. semifasciata, as in the earlier forms of other generic types, the distinctive generic 
features may not be, and usually are not, as fully developed as they become in species of later geological 
periods. 
f Beecher and Clarke. Development of some Silurian Brachiopoda, p. 25. Mr. Foerste has shown 
that the same conditions prevail in 8. patenta, as in 8. striata of the Clinton group of Ohio and other 
localities, and writes as follows: 
“ This shell begins its existence with the ventral valve convex and the dorsal concave. Later the ante¬ 
rior and lateral margins of the ventral valve become concave or turned up, and that of the dorsal valve 
convex or turned down. Since this is likewise true of Strophodonta striata, and only the exterior of these 
specimens is found, as a rule, these species would be difficult to distinguish, were it not for the fact that 
neither the convexity of the ventral valve, nor the point of strongest concavity of the dorsal valve, lies ever 
so near to the cardinal line in 8. patenta as in 8. striata. In other words, 8. patenta remained for a longer 
time a shell of simple curvature.”—Notes on Clinton Group Fossils, with special reference to Collections 
from Indiana, Tennessee and Georgia, by Aug. F. Foerste. • Proceedings of the Boston Society of Natural 
History, vol. xxiv, pp. 300, 301. 1889. 
See, also, Strophomena patenta; Geological Survey of Ohio ; Palaeontology, vol, ii, p. 115. 1875. 
Whether this species is a Strophomena or a Strophonella remains to be determined, as we do not yet know 
the exact character of the hinge. The best specimens obtainable do not furnish conclusive evidence of 
crenulations along this line, while they do show a broad open.delthyrium, with evidence of a small apical 
callosity, or deltidium. Should this species prove a Strophomena, it is the last of its race, remarkable in 
having reverted to a primitive or orthoid condition of deltidial structure. Nevertheless the facts above given 
are certainly of very great interest as showing the progressive generic development in certain forms of 
Brachiopoda, and in this case suggests the inquiry as to how far certain strophomenoid characters may 
have been carried forward into Strophonella. 
