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. disjunctus and its allies except in the 
general suppression of the external characters, the persistently shorter hinge 
and the occasional manifestation of dichotomous 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. concimus, Hall, which bears 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. concimus, Hall, 
S. Grieri, Hall, 
S. Orestes, Hall, 
S. Williamsi, sp. nov., 
S. Keoteuk, Hall, 
(S. Littoni, Swallow, 
S. hifurcatus. Hall, 
S. Leidyi, Norwood and Pratten, 
S. increbescens. Hall, 
S. opimus. Hall, 
Lower Helderberg group. 
Corniferous limestone. 
Lower Upper Devonian. 
Chemung group. 
Keokuk group. 
St. Louis group. 
St. Louis group. 
St. Louis and Chester groups. 
Chester group. 
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 Avhich are crossed by fine, closely set concentric lines 
each bearing a fimbria of short, simple spines. 
This peculiar type of structure is represented, as far a known, only by the 
species, 8. divaricatus. Hall, of the Upper Helderberg and Hamilton faunas. It 
is remarkable in all its characters and perhaps should be regarded as an isolated 
