GENEILA OF THE SUBORDER PENTAMEROIDEA 



169 



genus. It does not belong to Rhynchonella or Cam- 

 erophoria, since the sinus is situated on the dorsal 

 (? ventral) valve; moreover, from Rhynchonella as 

 well as from Enton'ut and Pentamerus it is clearly dif- 

 ferentiated in having a very large strong septum in 

 each valve. The only existing example was found in 

 a "Lower Silurian" (Ordovician) erratic boulder 

 thought to belong to the Jewe horizon of the Estonian 

 Ordovician. 



Hall and Clarke remark on the fact that the author 

 may have reversed the position of the ventral and dorsal 

 valves. The presence of a septum in each valve and 

 the occurrence of the shell in rocks of probable Trenton 

 age suggests a relationship of Branconia to Camerella. 

 However, in the absence of definite information re- 

 garding the nature of the interior and in the absence of 

 comparative material, the genus will stand for the 

 present. 



Genus PARASTROPHINELLA Schuchert 

 and Cooper 1931 



PI. 25, figs. 23, 25, 26; pi. 29, fig. 7 



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

 p. 248. 



Genoholotype. — Pentamerus reversus Billings 

 1857, Geol. Surv. Canada, Rept. Prog, for 1856, 

 p. 295 ; Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 

 pi. 63, fig. 11. 



Description. Exterior. — Shell subglobular in out- 

 line, hinge-line narrow, lateral profile biconvex, the 

 dorsal valve always the more convex and usually 

 arched somewhat over the ventral interarea. Anterior 

 commissure broadly uniplicate. Ventral interarea nar- 

 row, apsacline, beak curved slightly, delthyrium open, 

 so far as known. Dorsal interarea obsolete, beak in- 

 curved, umbo strongly arched. Surface costate, the 

 costje being faint or obsolete at the umbones as in 

 Camerella. Shell structure fibrous, impunctate. 



Ventral interior. — Like that of Camerella, with a 

 prominent spondylium duplex, the septal plates of 

 which are prolonged for some distance in front of the 

 spondylium. In one species and in some individuals of 

 other species the spondylium is sessile at the posterior of 

 the shell. 



Dorsal interior. — Like that of Anastrophta, with the 

 septal plates discrete or convergent only at their front 

 ends. Alse somewhat better developed than in Camer- 

 ella. Adductor scars anterior to the cruralium. 



Geologic range. — Upper Ordovician to Middle 

 Silurian of North America. 



Species 



Parastrophia divergens Hall and Clarke 1895 

 P. latiflicala Hall and Clarke 1895 

 P. multiflicala Hall and Clarke 1895 

 P. ops (Billings) 1862 

 /». rw/ff/d (Billings) 1857 



Distinguishing characters. — This new genus 

 differs from Camerella srnsu stricto chiefly in the 

 divergence of the septa of the dorsal valve, which do 

 not form a cruralium duplex. It thus forms, with 

 the other Camerellidx, a parallel scries with the 

 Gypidulina:. 



Discussion. — Hall and Clarke proposed the name 

 Parastrophia and typified the genus by Atrypa hemi- 

 plicata Hall. Little did they realize that they had 

 selected a genotype whose internal structure is identical 

 with that of Camerella volbortht Billings. However, 

 several of the later species usually placed under the old 

 name of Parastrophia prove to have features in the dor- 

 sal valve differing from those of Camerella and are 

 here placed under the new name Parastrophinella, 

 chosen to preserve a semblance of the old term. 



It is known from internal molds that the adductor 

 muscles of the dorsal valve of Parastrophinella were 

 situated entirely outside of the cruralium as in Camer- 

 ella. In this respect, Parastrophinella agrees with the 

 other members of the family. 



Genus ANASTROPHIA Hall 1867 



PL 25, figs. 14, 15, 19, 33-36, 38-42 



HaU, N. Y. State Cab., 20th Rept., 1867, p. 163. 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 224, 



pi. 63, figs. 31-38. 

 Horn. Brachymerus Shaler 1865 



Genolectotype (Hall and Clarke). — Pentamerus 

 verneuili Hall 1857, N. Y. State Cab., 10th Rept., 

 p. 104, figs. 1, 2. 



Description. Exterior. — Globular to subellipti- 

 cal; hinge-line straight, narrow; cardinal extremities 

 broadly rounded; lateral profile unequally biconvex, 

 anterior commissure uniplicate, the dorsal fold usually 

 being defined only in the anterior half of the valve. 

 Ventral interarea small and narrow, cleft by an open 

 delthyrium; dorsal interarea obsolete; beak curved, 

 commonly resorbed. Surface multicostate ; shell sub- 

 stance fibrous, impunctate. 



Ventral interior. — Teeth narrow, sharp, with deep 

 crural fossettes, the teeth forming a strengthening 

 ridge along the delthyrial margin; dental plates con- 

 vergent, making with the septal plates a spondylium 

 duplex which is sessile or nearly so at the posterior of 

 the shell and is supported in front by the median 

 septum. Scar of pedicle attachment located just be- 

 neath the dental ridges. 



Dorsal interior. — Notothyrial cavity deep; brachial 

 supports stout curved processes, carinate on their pos- 

 terior face, sloping laterally into a concave fulcral plate 

 which is attached to the inner shell wall. Sockets deep. 

 Septal plates nearly vertical lamell.-c, subparallel or con- 

 verging slightly toward the anterior. On the outside 

 of these plates and beneath the brachial supports are 

 winglike plates which are concave outward. In the 



