BRACHIOPODA. 247 



ian of the Hartz Mountains,* resembling P. galeatus in contour though some- 

 what shorter and more sharply costated. If the significance of this variability 

 be restricted to a specific or even more subordinate value, we shall have a group 

 of Silurian shells essentially equivalent to Barrandella, comprising such species 

 as P. galeatus, P. Roemeri, nom. propos. (= P. galeatus, Roemer, op. cit.), P. nucleus. 

 Hall, P. u?iiplicatus, Nettelroth, P. Sieberi, von Buch. All these shells are without 

 evidence of cardinal area or deltidial plates, have the plication of the surface 

 more strongly developed upon fold and sinus, and agree in the internal structure 

 of the pedicle-valve. As the designation Sieberella has been brought into use 

 for one of these species, we may take the liberty of broadening its significance 

 by basing it upon more stable characters than those selected by its author, and 

 applying the term to all shells of this type of structure, taking no account of 

 the specific variability in the internal structure of the brachial valve. 



Typical Swedish and English Silurian specimens of Pentamerus (Sieberella) 

 galeatus possess a peculiar surface sculpture consisting of very fine, irregularly 

 anastomosing concentric lines, and, in rare instances, a similar character is pre- 

 served in the Lower Helderberg specimens of the same species. In regard to 

 the various shells from the Devonian that are referred to P. galeatus by the 

 European palasontologists, it may be suggested that they are less likely to 

 represent this specific type than to indicate the presence, in those faunas, of 

 shells referable to the Devonian genus Gypidula. 



This group Gypidula, Hall, 1867, includes those galeatiform shells of the 

 second division which have a well-defined, true, cross-striated cardinal area, 

 and narrow, but erect or convex, incip- 

 ient deltidial plates. On the interior 

 the teeth are unusually strong, the 

 septum of the pedicle-valve very short, 

 the spondylium being free for most „ , ,P'^'JV' ■ ,-. 



r %^ o PentameriLS [Gypidula) comis, Owen, 



of its length. In the opposite valve a ..■ansver.o section in front or the short me.lian septum ..r 

 o r r the peihcle-valve; showing the form ol the spondyha. 



the dental sockets are distinct, the "'•' 



crural plates expanded nearly horizontally, being divided at their beginning 



* Kayser, Abhaudl. zur geol. Specialkarte von Preussen, etc., Band 2, heft 4, p. 156, pi. xxvii, 

 figs. 1-9, 13. 1878. 



