BRACHIOPODA. 27 



These species suggest the short-winged, rounded forms of <S. arenosus occur- 

 ring in the Oriskany sandstone of Cumberland, Maryland, and the type of 

 structure is not dissimilar to that of S. disjundus and its allies except in the 

 general suppression of the external characters, the persistently shorter hinge 

 and the occasional manifestation of dicliotomous ribs. 



(e) Orestes-type. Shells of small size, moderately extended on the hinge; 

 lateral plications simple and usually few in number ; fold and sinus angular and 

 with few plications, of which the median members are much the strongest. 

 Surface usually ornamented by fine, hair-like, often granulous radiating lines. 



This group makes its appearance as early as the fauna of the Lower Helder- 

 berg group, in the species S. concinnus, Hall, which Ijears a few faint median 

 plications, usually observable only near the anterior margins of the valves. In 

 the Corniferous limetone, S. Grieri, Hall, is a similar form. With the appear- 

 ance of the early Carboniferous faunas the type became abundantly exemplified, 

 but disappeared in the Coal measures. It is represented by the following 

 species : 



S. concinnus, Hall, Lower Helderberg group. 



S. Grieri, Hall, Corniferous limestone. 



jS. Orestes, Hall, Lower Upper Devonian. 



S. Williamsi, sp. nov., Chemung group. 



iS. Keokuk, Hall, Keokuk group. 



S. Littoni, Swallow, St. Louis group. 



S. bifurcatus, Hall, St. Louis group. 



iS. Leidyi, Norwood and Pratten, St. Louis and Chester groups. 



S. increbescens. Hall, Chester group. 



S. opimus. Hall, Coal measures. 



(f ) Divaricatus-type. Species with hinge not extended, low fold and sinus ; 

 numerous fine dichotomous lateral plications not differing in size from the 

 median plications, all of which are crossed by fine, closely set concentric lines 

 each bearing a fimbria of short, simple spines. 



This peculiar type of structure is represented, as lar a known, only by the 

 species, S. divaricatus, Hall, of the Upper Helderberg and Hamilton faunas. It 

 is remarkable in all its characters and perhaps should be regarded as an isolated 



