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/njcwpme/ = Delthyris, Dalman, 1828, sensu stricto ; those species in 

 which the concentric fimbriae are made up of short, simple, hollow spines. 

 Those 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. odocostatus, Hall, of the Lower 

 Helderberg group, S. arredtts, 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. crisptts-type begins with the S. 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, S. Canandaigua* from the Hamilton group of New 

 York, and possibly, S. urbanus, Calvin, from a corresponding horizon in Iowa, 

 the line terminating in (b) S. 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 S. laevis-type) 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 



 Kor deacription of this species see Supplement to this volume. 



t See observations on, and illustrations of Spirifer bicostatus and var. petilux, and young of S. crispus 

 and var. simplex, in Memoirs New York State Museum, vol. i, No. 1, pp. 76-77, pi. vi, figs. 1-7. 1889. 



