756 Report oF THE STATE GEOLOGIST. 
2. Duplicispinet = Ruricunrarta, McCoy, 1844; those species 
in which the fimbrize are composed of large, compound, hollow 
spines, often or always with lateral branches. ach spine is 
divided medially by a vertical septum, and along this line the 
spine is depressed exteriorly, giving it a double-barreled appear- 
ance; from each lateral margin are given off at regular intervals 
short spinules at right angles to the main spine. (S. jimbriatus, 
Conrad; S. subundiferus, Meek and Worthen; S. Airtus, White 
and Whitfield; S. dineatus, Martin.) 
Section IV. Aprrerturatir. (Plate 26, figs. 1-10.) Typical ex- 
amples, S. aperturatus, Schlotheim, S. disjunctus, Sowerby 
Si striatus, Martin. 
Forms with plications on the fold and sinus. 
This may be regarded as the typical group of Spirifers as it 
includes the type-species, S. striatus, Martin. Furthermore it is 
the most richly represented in species and at the same time is a 
most compact association, not presenting any substantial varia- 
tions. Its members are strongly impressed with the typical spir- 
iferoid characters which are maintained throughout its existence, 
the group terminating abruptly at.the close of palzozoic time. 
In internal structure variations are slight and unimportant. The 
dental lamellz are, as a rule, inconsiderably developed, and there 
is no median septum in either valve. 
(a) Disjunctus-type. Forms with well-developed fold and sinus, 
elongate hinge and elevated cardinal area; lateral plications 
simple, median plications dichotomous or intercalary. 
(Spirifer arenosus, Conrad, Oriskany sandstone ; Spirifer wni- 
cus, Hall, Corniferous limestone; Spirifer Whitneyi, Hall, Lower 
upper Devonian; Spirifer disjunctus, Sowerby, Chemung group.) 
A subordinate division of this section is the 
(1) Hungerfordi-type (—Cuoristirrs, Fischer de Waldheim, 
1825), in which the fold and sinus are low, often obsolescent, the 
outline suborbicular and the cardinal area compressed laterally 
and incurved ; dental lamellz prominently developed. This type 
is represented by S. Hungerfordi, Hall, of the lower Upper 
Devonian. 
(b) Striatus-type. Forms having a great number of duplicate 
lateral plications, well developed, rarely acuminate fold and sinus, 
and narrow, usually extended cardinal area. 
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