BRaAcHIOPODA. 157 
(S. striatiformis, Meek, Waverly sandstone ; S. Loganz, Hall, 
Keokuk group; S. striatus, Martin; S. Marcowi, Waagen.) 
(c) Imbrex-type. Alate, mucronate shells, with narrow cardi- 
nal area, fine, simple (very rarely duplicate) lateral plications, 
the plications on fold and sinus being of about the same size as 
the rest. The surface is frequently lamellose. 
(S. Mewberryi, Hall, Waverly group; S. Marionensis, Shumard, 
Waverly and Choteau groups; S. biplicatus, Hall, Kinderhook 
group ; S. ¢mbrex, Hall, Burlington limestone.) 
(d) Suborbicularis-type. Forms with suborbicular outline, 
broad, low and usually simple lateral plications; the median 
plications are few and indistinct. 
(S. suborbicularis, Hall, Kinderhook-Keokuk groups; S. subcar- 
dviformis Hall, St. Louis group.) 
(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 a few plications, of which the 
median members are much the strongest. Surface usually orna- 
mented by fine, hair-like, often granulous radiating lines. 
(S. concinnus, Hall, Lower Helderberg group; S. Grieri, Hall, 
Corniferous limestone; S. Orestes, Hall, lower Upper Devonian ; 
S. Keokuk, Hall, Keokuk group; S. optimus, 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. 
(S. dwaricatus, Hall, Upper Helderberg and Hamilton groups.) 
Section V. Osrionati. (Plate 27, figs. 1-17.) Typical examples, 
Spirifer ostiolatus, Schlotheim, S. Oweni, Hall. 
Forms with the median fold and sinus without plications. 
(S. pereatensus, Meek and Worthen; S. macrothyris, Hall, 
Corniferous limestone; S. Marcyi, Hall; S. audaculus, Conrad ; 
S. granulosus, Conrad; S. Parryanus, Hall; S. asper, Hall, 
Hamilton group.) 
These species, in the degree of plication of the sides and the 
._ development of the muscular scars, closely resemble the members 
of the foregoing group. As a rule the Ostiolati are stouter 
shells, shorter on the hinge and more yentricose than the 
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