292 PALAEONTOLOGY OF NEW YaRK. 



In Strophonella as in Stropheodonta, the earlier representatives possess 

 some of the generic characters in an incipient condition. Species are not 

 known to occur in faunas earlier than the Clinton, where this group is repre- 

 sented by (?) S. patenta and S. striata. The latter species is continued into the 

 Niagara group, where it is associated with i'. semifasciata, at Waldron, Indiana. 

 The genus is represented in Europe by S. euglt/pha and S. funiculata. 



Strophonella striata, Hall, of the Niagara fauna, has a deltidiuni highly devel- 

 oped in the immature stages of the shell, a prominent deltidial callus, and a 

 short row of cardinal denticulations ; it is, in fact, in these respects a reversed 

 Brachyprion, bearing precisely the same relation to Strophonella in its fuller 

 development as that group does to Stropheodonta. The species also presents 

 some differences from the later Strophonellas in its internal characters. In 

 the pedicle-valve the muscular area is not enclosed, but its lateral margins are 

 bounded by two curved ridges continued from the dental plates. In the brach- 

 ial valve there is a similar arrangement produced by the continuation of the 

 crural plates, l)ut the muscular impressions are extremely faint, the radial 

 markings of the surf;ice extending quite to the base of the cardinal process. 



Should it be considered useful to recognize the incipient and progressive 

 features of the species S. striata, and probably jS. patenta, towards a full manifesta- 

 tion of generic characters, and distinguish them from Strophonella in its more 

 mature condition of development, the term Amphistrophia may prove expressive 

 of their apparent double relation as shown in the young and mature shells. 



In the other Silurian species of Strophonella the deltidiuni is more or less 

 prominently developed as a convex plate, but in the Devonian these parts assume 

 the character they possess in Stropheodonta demissa. In the species of the Lower 

 Helderberg and Devonian the muscular area of the pedicle-valve is deeply 

 impressed and strongly elevated at the margins, less like that of Stropheodonta 

 demissa than that of the later Stropheodontas. In the Lower Helderberg fauna 

 are the species S. Headleyana, Hall, S. cavumbona. Hall, iS. pundulifera, Conrad, 

 S. Leavenworthana, Hall, which possess all the typical features of the genus. It is 

 probable that Strophomena radiata and S. Conradi, Hall, belong to the same genus, 

 but their interior characters are not fully known. In the Upper Helderberg we 



