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 Kept. 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 fii-st-named species in the list it will pi-obably be i-eg-arded by students as the " type " 

 of the g-enus, 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 >S'. semifasciata, S. Leavenwai-thana, S. cavumbona and S. ■punctulifiira (the last two probably 

 identical), of the Lowei' Helderberg group, aS". ampla, of the Ujiper Helderberg group, iS". reversa, and 

 >S'. ccelata, of the Chemung group ; the interior characters of S. semifasciata were less known than in any of 

 the others, and it was included in the gi'oup fi'<iin its resupinate form and the ch.aracter of the cardinal area, 

 deltidium, etc. In (S. semifasciata, as in the eaiiier 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. 



t Bbecher and Clarke. Development of some Silurian Brachiopoda, p. 2.1. Mr. Fobbste has shown 

 that the same conditions prevail in S. patetita, as in fi. 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 ditficult 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 S. patetita as in <S. striata. In other words, S. patenta remained for a longer 

 time a .shell of simple curv-ature." — Notes on Clinton Group Fossils, with special reference to Collections 

 from Indiana, Tennessee and Georgia, by Aug. F. Fobrste. Proceedings of the Boston Society of Natural 

 History, vol. xxiv, pp. 300, 301. 1889. 



See, also, Stropliomena patenta ; Geological Sni-vey 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 lin^, while they do show a broad open delthyrium, with evidence of a small apical 

 callosity, or deltidium. ShouUl this species prove a Strophomena, it is the last of its race, remarkable in 

 having reverted to a primitive oi' orthoid condition of deltidial structure. Nevertheless the facts above given 

 are certainly of very great intei'est as showing the pi'ogressive generic development in certain forms of 

 Brachiopoda, and in this cose suggests the inquiry as to how far c-ertain strophomenoiil characters may 

 have been carried forward into Strophonella. 



