176 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Genus PENTAMERELLA Hall 1867 



PI. 26, figs. 4, 13-17, 19,20; t. fig. 32 



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

 Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, pp. 242, 

 245, pi. 71, figs. 21-29. 



Genolectotype (Hall and Clarke). — Atrypa 

 arata Conrad 1841, 5th Rept. N. Y. Geol. Surv., 

 p. 55. 



Description. Exterior. — Outline subtriangular to 

 subpentagonal ; hinge-line narrow; cardinal extremi- 

 ties rounded ; lateral profile biconvex, the ventral valve 

 having the greater convexity. Anterior commissure 

 uniplicate; dorsal fold usually low, in some species 

 nearly obsolete. Ventral interarea narrow, curved, 

 apsacline, beak incurved strongly, umbo swollen; del- 



P. intrdineata (Winchell) 1 866 

 P. missouriensis Branson 1922 

 P. favilionensis (HaW) 1860 



Distinguishing characters. — Externally Pen- 

 tamerella most closely resembles Gypidula, but differs 

 in having the fold on the dorsal valve. Internally, it 

 is similar to SieberelLa in having the septal plates unite 

 with a low septum to form a cruralium. 



Discussion. — Of interest in Pentamerella is the ex- 

 tremely short septum of some of the species. In shells 

 referred to P. favilionensis , from Moreland, Kentucky, 

 the septum may be confined to the very posterior of 

 the shell as a short rib. In the brevity of its sep- 

 tum Pentamerella represents an advanced generic 

 development. 



o o 6 6 <5 O 



I 



8 



Fig. 32. — Pentamerella aff. P. ■favilionensis (Hall), Alpena, Michigan. Of prime interest in these serial sections is 

 the extreme brevity of the median septum in the ventral valve. It was apparently a ridge on the posterior wall of the 

 shell. X 1 . Distance from beak: 



1 — 1.9 mm. 

 2—3.9 

 3—4.2 

 4—4.9 



5 — 5.5 mm. 



6— 6.9 



7— 7.6 

 8—11.25 



thyrium with incipient deltidial flates. Dorsal inter- 

 area obsolete, beak curved under that of the ven- 

 tral valve. Surface usually multicostate, occasionally 

 smooth. Shell substance fibrous, impunctate. 



Ventral interior. — Teeth narrow and sharp; spon- 

 dylium duplex shallow; floor of spondylium longi- 

 tudinally striated, supporting septum short; intern.il 

 umbo-lateral ovarian spaces pustulose. 



Dorsal interior. — Notothyrial cavity deep; plates 

 supporting brachial processes broad and flat, supported 

 by thin curved plates which unite in the mid-line of 

 the shell to form a cruralium that in some specimens is 

 supported by a low duplex septum. In the apex is a 

 shallow pit divided by a low ridge ; the two pits are the 

 areas of the diductor scars. 



Geologic range. — Devonian (Middle and Up- 

 per) of North America. 



Species 



Pentamerella arata (Conrad) 1841 



P.dubia (Hall) 1860 



P. juhoneiisis Branson 1922 



Booker^* has recently published figures of "Pentam- 

 erella" molongensis Mitchell, which are obviously not 

 of this genus. In this form the septal plates are widely 

 divergent as in Clorinda or Gyfidula. The same 

 is true also of "Pentamerella" sublinguifer Maurer, 

 figured by Leidhold.'^ In this shell the dorsal septa 

 are wide-spread and the form is certainly not a Pen- 

 tamerella. The figures suggest Clorinda, as the shell 

 is quite smooth. 



Hall and Clarke say that in Pentamerella "there 

 are occasionally evidences of lateral, erect or convex 

 growths upon the margins of the delthyrium, which 

 may be interpreted either as remnants of a resorbed 

 convex deltidium, or as highly accelerated secondary 

 deltaria" (pp. 341, 342). We have also seen these 

 plates, and interpret them as incipient deltidial plates. 



"Jour. Proc. Roy. Soc. N. S. Wales, vol. 60, 1926, pp. 

 140-142. 



" Abh. preuss. geol. Landcsanst., N. F., Heft 109, 1928, 

 p. 57, pi. 4, fig. 14, t. fig. 25. 



