246 PALEONTOLOGY 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- 

 dylium is formed j the septa supporting the crural plates resting directly on the 



FIO. 17S. Fio. 176. 



Fig. 175. Tranavene section of Pentamenu [SiebereUa) gaieattu, near tho beaks, the pedlole-vAWe being uppermost; 



showing the discrete septa of tbe brachial valve. (C.) 



Fig. 17<. Pentamtrvt {SUbtrdta) Sitberi, von Uucb. Transverse section, showing the fomi 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 (Ehlert, 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 F^ and in the Hercyn- 



• Die Silurische Fauna des weatlicben Tennessee, p. 73, pi. v, fig'. 11. 



t In Fiscbbr'k Manubl de Conchyliologie, p. 1311. 1887. 



J Babrakdb, Systdme Silurien, vol. v, pis. xxi, Ixxvii-Isziz, cxviii, cxix. 



