GEMS FROM PRIVATE COLLECTIONS. 313 



GEMS FROM PRIVATE COLLECTIONS. 



SPIRIFERA COWOLUTA. 



(From tlie Carboniferous Limesto7ie of Tliorneley. In the collection of 

 J. RoFE, Esq., F.G.S.) 



By Thomas Davidson, Esq., F.R.S., F.G.S., etc. 



Since some pages of yonr valuable magazine are occasionally devoted 

 to the description and illustration of " Grems from Private Collec- 

 tions," allow me to offer a drawing and some observations on 

 Spirifera convoluta, Pliillips, one of the most remarkable of om- 

 British carboniferous spirifers, and which will serve at the same time 

 as a small supplement to the paper I published in the eleventh num- 

 ber of The Geologist. 



At page 35 of my "MonogTaph" I described 8p. convoluta as a 

 distinct species, but subsequently I thought that it might, perhaps, 

 be a very transverse or exceptional condition or variety of S'X). hisul- 

 cata, Sowerby. 



Experience teaches us that individuals of the same species may 

 assume great differences in shape ; that a Sjoirifera, for example, which 

 in its normal condition is about as wide as long, may at times exceed 

 these proportions very considerably iil one or the other direction, and 

 that we are often too prone to seize upon similar variations as 

 pretexts for the creation of so termed species. 



Sjpirifera hisulcata is one of these variable species which, in its 

 usual condition, presents a somewhat semi-circular or sub-rhomboidal 

 shape, rather wider than long, with its valves almost equally convex 

 or deep ; the hinge-line is straight, and either shorter or as long as 

 the greatest width of the shell ; when shorter, its angles are rounded ; 

 and when otherwise, they project at times to a considerable extent, 

 with angular terminations. The area is of moderate height, and the 

 beak sometimes so much incurved as almost to touch the umbone of 

 the opposite valve. Each valve is ornamented by from twenty to 

 forty ribs, which vary in width and degree of projection in different 

 specimens : in some they are rounded and flattened, while in others 

 they are sharply angular ; the sinus is of moderate depth, and the 

 mesial fold rounded and generally divided into three principal por- 

 tions by two deep sulci ; but each of these are in their turn more or 

 less subdivided into two or more ribs of lesser projection. The 

 whole surface of the shell in well-preserved specimens is also covered 

 with close concentric lines of growth, which gives to the surface a 

 kind of imbricated appearance. 



