246 
PALjEONTOLOGY of new YORK. 
development of the median septum of the pedicle-valve, while the spondylium 
is moderately long, and is free for fully two-thirds its length. In the brachial 
valve of typical examples from the Gotland and Wenlock limestones no spon- 
dyliuni is formed; the septa supporting the crural plates resting directly on the 
Fig’. 175. Transverse section of Pentamerus (Sieherella) galeatus, near the beal<s, the pedicle-valve being uppermost; 
showing the discrete septa of the brachial valve. (c.) 
Fig. 176. Pentamerus (Sieherella) Sieheri, von Buch. Transverse section, showing the form of the spondylia. 
(C.) 
surface of the valves. To what extent the latter feature varies among the 
European Silurian specimens of this species we do not know, but in the 
American representatives of this type of structure, the variability in devel¬ 
opment of these crural plates is very apparent, and confirms the opinion 
already expressed, that the union or independence of the septa of the 
brachial valve is not a feature of generic importance. There are two American 
shells which are currently referred to this species; the one from the Lower 
Helderberg fauna of New York has the closest similarity to the English shells in 
all points of structure, and in this one the septa in question invariably remain 
independent. A smaller shell also occurs in the Upper Silurian fauna of Perry 
county, Tennessee, which derives its name from an early identification by the 
late Professor Ferdinand Roemer.* In this shell, however, these septa appear 
to be frequently, if not invariably, convergent, forming a spondylium resting 
upon a median septum. In view of this and similar evidence which has already 
been cited, it seems impossible to follow CEhlert, who has proposed to restrict, 
under the generic name Sieberella,! shells of this type in which these plates 
are united and supported. This name has been based upon the species P. Sieberi, 
von Buch, a shell which abounds in the Bohemian Etage ^ 2 ^ and in the Hercyn- 
* Die Silurische Fauna des westlicben Tennessee, p. 73, pi. v, fig. 11. 
t In Fischer’s Manuel de Conchyliologie, p. 1311. 1887. 
I Barrande, Systeme Silurien, vol. v, pis. xxi, Ixxvii-lxxix, cxviii, cxix. 
