GENERA OF THE SUBORDER PENTAMEROIDEA 



175 



and this was also Leidhold's solution (1928).''' In 

 1913,'^ however, the former author indicated S'leber- 

 ella as a subgenus of Gyfidula and in the Zittel- 

 Eastman Text-book of Paleontology both were given 

 independent rank. Recently Belanski (1928), in a 

 study of the Iowa Upper Devonian pentamerids, notes 

 the internal differences mentioned by Hall and Clarke 

 and also the fact that Sicherella sieberi actually has a 

 cardinal area. The latter fact has been corroborated 

 by the present writers. This heretofore superficial dis- 

 tinction therefore fails, with the result that the chief 

 difference between the genera lies in the dorsal interior. 

 It is just as correct to assume a line of reasoning 

 diametrically opposed to that of Hall and Clarke: 



commissure sulcate; ventral fold usually defined only 

 on the front half of the shell. Ventral interarea rather 

 wide for the subfamily, curved, apsacline, delthyrium 

 open, beak incurved strongly, umbo inflated. Dorsal 

 interarea obsolete, beak incurved, umbo swollen. Sur- 

 face multicostate ; shell substance fibrous, impunctate. 



Ventral interior. — Like Gyp'ulula. 



Dorsal interior. — The tripartite character of the car- 

 dinalia is essentially the same as that of Gypidula, but 

 the septal plates, instead of uniting directly with the 

 floor of the valve, unite with a low median septum. 



Geologic range. — Silurian and Devonian of 

 North America and Europe. 



(D (t^ 







QX) 



\J 



\j 



Fig. 31. — Sieberella sieberi Barr. Sections showing internal structure, 

 dorsal septa to form a single plate. Distance of sections from beak: 



1 — 1.6 mm. 

 2—2.8 

 3—3.5 

 4 — +.0 



Of greatest significance is the union of the 



5 — 4.8 mm. 



6—7.9 



7—9.2 



the internal structures can be seized upon as diagnostic, 

 the cruralium of Sieberella distinguishing that genus 

 from Gypidula, which has divergent lamellae. The 

 striking external resemblance in form we therefore 

 attribute to homoeomorphy. 



Genus SIEBERELLA Oehlert 1887 

 PI. 26, figs. 25, 30, 33; t. fig. 31 



Oehlert in Fischer, Man. Conch., 1887, p. 1311. 

 HaU and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, pp. 241, 

 246, pi. 72, figs. 4, 5, t. fig. 176. 



Genoholotype. — Pentamerus sieberi Von Buch in 

 Barrande 1847, Brach. Sil. Schichten Boehmen, p. 

 103, pi. 21, figs. 1, 2. 



Description. Exterior. — Outline galeatiform as 

 in Gypidula, hinge-line faintly arcuate; cardinal ex- 

 tremities rounded. Lateral profile biconvex, the ven- 

 tral valve usually with the greater convexity. Anterior 



"Op. cit., p. 50. 



*' Marj'land Geol. Surv., Lower Devonian, p. 342. 



American Species 



Sieberella emarginata Belanski 1928 



S.insolita Belanski 1928 



S. roemeri Hall and Clarke 1893 (in part) 



European Species 



Sieberella multistriata Roemer 



S. sieberi (V. Buch) 1847 



S. sieberi rectifrons (Barrande) 1879 



Discussion. — Hall and Clarke restricted the name 

 Sieberella to shells not having a cardinal area on the 

 ventral valve, this being their sole distinction. A re- 

 study of the genotype, however, shows that it possesses 

 a well defined area. Nevertheless, there is a funda- 

 mental internal difference, the convergence of the 

 septal plates to meet a median septum. This type of 

 structure is not common and is restricted seemingly 

 to rather transverse shells which are costate over nearly 

 the whole surface. Sieberella has the fold restricted to 

 the ventral valve and is thus a homoeomorph of 

 Gypidula. The two genera form a striking example 

 of internal variation combined with a stable external 

 expression. 



