GENERA OF THE SUBORDER PENTAMEROIDEA 



179 



This development is so different from Pfntainerus 

 that we are distinguishing it as a new genus under the 

 name Pe'titamerouifs. 



The most interesting internal feature of Pintam- 

 erus is the long brachial process which reaches to the 

 anterior end of the spondylium. The two processes 

 appear to remain separate, judging by serial sections 

 prepared by us. 



Genus PENTAMEROIDES Schuchert and 

 Cooper 1931 



PI. 27, figs. 13, 14; pi. 28, figs. 19, 22; t. fig. 27 



Schuchert and Gx)per, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 248. 



Genoholotype. — Pentamerus suhrectus Hall and 

 Clarke, 1893, Pal. N. Y., vol. 8, pt. 2, p. 238, pi. 69, 

 figs. 2, 3, 8-10. 



Distinguishing characters. — Externally the 

 members of this genus are precisely like Pentamerus, 

 but internally there is a cruralium in the dorsal valve 

 that in some specimens is sessile for a short distance 

 at the posterior but is elevated at the front. In some 

 species, however, the cruralium is supported by a promi- 

 nent median septum for its whole length. In all of 

 the species the dorsal septum is rather long and is not 

 uncommonly longer than the ventral one. A careful 

 study of internal molds, a common mode of occurrence 

 of the genus, shows that the adductor muscles were 

 attached to the upper surface of the cruralium, for no 

 trace of muscle scars has been observed on the floor 

 of the valves. 



We have selected Pentamerus suhrectus as the type 

 of the genus. This is a well defined species charac- 

 terized by its shouldered dorso-lateral extremities. 

 There are a number of unnamed species occurring in 

 the Wisconsin dolomites, especially at Bailey's Harbor. 

 Here occurs a flat form much like the Clinton P. ob- 

 longus, but another type externally nearly identical 

 with Pentamerus cylindricus is also known. 



Discussion. — Pcntameroides has a wide distribution 

 in Silurian strata, being fairly common in Iowa and 

 Wisconsin. It was also found associated with a small 

 Stricklandia in the Silurian beds on the shore of the 

 Bay of Chaleur at Black Point postoffice. New Bruns- 

 wick. Some of the shells called Pentamerus borealis 

 by European writers are of this type. It is thus clear 

 that there must be a drastic revision of the species of 

 Pentamerus s»nsu lata. 



Geologic range. — Widely distributed in the Silu- 

 rian of North America and Europe, but the species 

 are not at all worked out, being referred to other 

 genera and mainly to Pentamerus. 



American Species 

 P.tubrectus Hall and Clarke 1893 



Genus LISSOCOELINA Schuchert and Cooper 

 1931 



(Gr. lissosy smooth ; ko'ilia, belly) 



PI. 28, figs. 7, 14, ? 13, ? 15, .? 16, ? 21 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 248. 



Genoholotype. — Pentamerus fergibbosus Hall 

 and Whitfield 1875, Pal. Ohio, vol. 2, p. 139, pi. 7, 

 figs. 10, 11. 



Distinguishing characters. — This name is pro- 

 posed for smooth shells having the external form of 

 Conc/iuiium in profile and outline and the internal 

 structure of Pentamerus. It represents an offshoot 

 from the Pentamerus line in the Middle Silurian. The 

 genotype is perhaps the best known example. The 

 ventral valve is elongated and the ventral beak is arched 

 over that of the dorsal valve as in C onchid'tum. In 

 Pentamerus sensu stricto there is not the great develop- 

 ment of the beak and the strong arching of the valves. 

 Even in the young of "fergibbosus" the same shape 

 of the adult is faithfully preserved and forbids any 

 relationship with Pentamerus s. s. In the ventral inte- 

 rior the median septum is extremely long and in the 

 dorsal valve the septa are rather close together and 

 subparallel as in Pentamerus. Externally the shell 

 preserves no trace of radii. 



We are placing in this association, but somewhat 

 doubtfully, P. maquoketa Hall and Clarke. In this 

 species the ventral septum is short and in this respect 

 suggests Harfidium but otherwise agrees fairly well 

 with that of Lissocoelitia. 



Subgenus CAPELLINIELLA Strand 1928 



Strand, Arch. Naturgeschicht., Berlin, vol. 92, A8, 1928, 



p. 38. 

 H.ill and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 248, 



pi. 70, figs. 6-14. 

 Horn. CafelliHia Hall and Clarke 1893. 



Genoholotype. — C. mh-a (Hall and Clarke) 

 1893. 



Description. Exterior. — Subpentagonal in out- 

 line; hinge-line exceedingly narrow; cardinal ex- 

 tremities obsolete. Anterior commissure rectimargin- 

 ate. Lateral profile unequally biconvex, the dorsal 

 valve always with the greater convexity. Ventral 

 interarea exceedingly narrow, almost obsolete. Del- 

 thyrium open. Dorsal interarea obsolete, beak strongly 

 curved, umbo swollen. Surface marked by obscure 

 radiating costa; and a broad low fold on each valve as 

 in Pentamerus. Shell substance fibrous, impunctate. 



Interior. — Essentially as in Pentamerus. 



Geologic range. — Silurian of Wisconsin. 



Discussion. — Specimens of this subgenus appear to 

 be rather rare and restricted geographically to Wis- 



