BRACHIOPODA. 
19 
sometimes notably developed; a low median septum may exist in the pedicle- 
valve. Surface covered with concentric rows or fringes of fine spines. 
This group is susceptible of the following subdivision : 
1. f/nzmpme/= Delthyris, Dalman, 1828, se?isu stricto; those species in 
which the concentric fimbriae are made up of short, simple, hollow spines. 
These are the early fimbriate species, the type of structure not extending, so 
far as now known, beyond the Devonian. The shells are distinguished from 
the other fimbriate Spirifers by their more extended and more distinctly pli¬ 
cated surface and the prominent, often sharply developed fold and sinus. Their 
more characteristic representatives in the American Palaeozoic are Spirifer 
crispus, Hisinger, and var. simplex, Hall, of the Niagara faunas; S. Vanuxemi, 
Hall, of the Tentaculite limestone, S. Saffordi, S. octocostatus, Hall, of the Lower 
Helderberg group, S. arrectus, S. tribulis, Hall, of the Oriskany sandstone and 
S. duodenarius, Hall, of the Corniferous limestone. 
This may be termed the (a) S. crispus-type. 
An interesting series of forms which has had a parallel development with the 
S. crispus-type begins with the <8. bicostatus, Vanuxem, and var. petilus. Hall, in 
the Niagara group, is represented by S. modestus. Hall, in the Lower Helderberg 
group, by a new species, <8. Canandaiguoe* from the Hamilton group of New 
York, and possibly, urbanus, Calvin, from a corresponding horizon in Iowa, 
the line terminating in (b) <8. laevis. Hall, from the lower Portage shales at 
Ithaca, N. Y. These are all unicispinate, but have the short hinge, subcircular 
outline and obsolescent plications characterizing the duplicispinate group of the 
Fimbriati. It is important to observe that this little series has, with reference 
to the main line of development of these forms, an expression of immaturity, 
the low plications and short hinge being features indicative of such a condition 
among those species.f With the exception of S. laevis, all the members of this 
subordinate group (the 8 . laevis-iypo) are quite small; the Portage species, how¬ 
ever, is one of the largest of fimbriate Spirifers, while its development, though 
abundant in individuals, is sharply localized. It stands out prominently as a 
* For description of this species see Supplement to this volume. 
t See observations on, and illustrations of Spirifer hieostatus and var. petilus, and young- of S. crispus 
and var. simplex, in Memoirs New York State Museum, vol. i, No. 1, pp. 75-77, pi. vi, figs. 1-7. 1889. 
