﻿80 



BRITISH DEVONIAN BRACHIOPODA. 



I. Str. arachnoidea, Phil. = sub-arachnoidea, Vern. 2. Sir. crenistria, Phil. = 0. peel en, 

 M'Coy. 3. Str. senilis, Phil. = 0. umbraculum, De Kon. 4. Str. Sharpei, = 0. umbracu- 

 lum, Portlock. 5. Sir. umbraculum, Schloth., as distinct species ; but I cannot myself 

 admit so great a number, and it must remain a question for further research, whether 

 even two of these {Str. umbracidum and Str. crenistria) may be retained as distinct species. 



In 1855, Dr. Sandberger (in his work on the Devonian Fossils of Nassau) makes use of 

 the designation Orthisina crenistria for the Devonian as well as the Carboniferous shell. 



Prom the above, and from several other authors' statements, it will be seen that some 

 palaeontologists have considered Str. crenistria a synonym of Sir. umbraculum, while others 

 would retain the two as distinct species ; and it was while labouring under this uncer- 

 tainty that in my description of the Carboniferous form I made use of the designation 

 crenistria, and am still undecided whether the Devonian form is in reality a distinct 

 species. Str. umbraculum, as found in the Devonian rocks, is very variable in its shapes, 

 just as is the case with the Carboniferous Street, crenistria. It has been affirmed by some 

 palaeontologists that in Str. umbraculum the shell is always concavo-convex, while the 

 ventral one is slightly concave near the margin in all fully developed or adult individuals, 

 and that on both valves the striae are strongly indented; the area always regular and 

 narrow ; and it was subsequently proposed by MM. A. D'Orbigny, Bouchard, and others, 

 to distinguish, by the designation of Strep. Bevonicus} those bi-convex specimens, which 

 are, on the contrary, very inconstant in their external form, both valves being convex, 

 their striae smooth and not strongly marked, while the area assumes every kind of shape, 

 both in height and width, being often irregularly twisted, and wider than long on one side 

 than on the other; the beak curved backwards or inclined to one or to the other side, 

 while nothing appears regular in the shell. Str. Bevonicus occurs in the Upper Devonian 

 limestone of Ferques, in France, but has by some palaeontologists been considered a mere 

 variety of the shell under description, and which they have at times designated by the 

 name of Orthis umbraculum, var. senilis. 



The minute examination I made of a multitude of specimens of Str. crenistria and 

 Str. senilis led me to the conclusion that both forms passed by insensible gradation into 

 one another, and consequently belonged to a single species ; a view subsequently corrobo- 

 rated by several local observers who had been able to study in situ a great many speci- 

 mens derived from the same bed and locality ; but not having been able to assemble a suf- 

 ficient number of examples of the Devonian Str. umbraculum and Bevonicus, I cannot give 

 out any positive opinion as to their distinctiveness, or to assert that they are mere varieties 

 of a single species. 



A great difference is also observable in the respective number and strength of the striae 



1 Leptcena Devonica, d'Orb. Prodrome, vol. i, p. 90, 1849 = Orthis crenistria, var. Bevonica, &c, 

 Keyserling. Geognost Beobacht, p. 221, pi. vii, fig. 7, 1.849. Russie Septentrionale, Ishma ; France, 

 Ferques. 



