STROrilOMENA. 128 



Mountain limestone division. It is not rare in the Carboniferous slate, in the parish of 

 Ballyseedy, county Kerry. Mr. Kelly mentions Kino-, Carrens, Millecent, and Hallina 

 county, among the Irish localities ; the variety " distorta " being very abundant, especially 

 in the county of Kildare. 



It is also a common species in the Carboniferous rocks of many foreign countries, very 

 large and fine examples occurring at Vise and Tournay, in Belgium. 



The Silurian and Devonian localities will be mentioned in the monographs treating 

 of the form belonging to those epochs. 



Sub-Ge?nts — Streptoriiynchus, King. 1850. 



The shells composing this sub-genus are closely related to Slrophomena ; they are 

 usually semicircular-convex or concavo-convex, externally striated and interstriated ; the 

 ventral valve possessing a prolonged and oftentimes bent or twisted beak. (See Part IV, 

 p. 29, of the present volume.) 



Many are the so-termed species that have been described as occurring in the Carbo- 

 niferous rocks of this and other countries, but so exceedingly variable are these shells, and 

 so intimately do they all appear connected and linked together by intermediate and insen- 

 sible graduations of shape, that it becomes most puzzling and difficult to determine how 

 far we may be permitted to limit the extent of variation, or to determine what shapes 

 ought to be separated or combined under a single species. Darwin considers "the term 

 species as one arbitrarily given for the sake of convenience to a set of individuals closely 

 resembling each other, and it docs not essentially differ from the term variety, which is 

 given to less distinct and more fluctuating forms ; that the term variety again, in comparison 

 with mere differential differences, is also applied arbitrarily and for mere convenience 

 sake." And further on the same authorobserves, that " no one can draw any clear dis- 

 tinction between individual differences and slight varieties, or between individual differences 

 or more plainly marked varieties and sub-species and species." ' And how often are we not 

 too prone to solve a difficulty in the way of identification, by at once cutting the Gordian 

 knot, and arbitrarily fabricating a new species, without seeking to determine or to trace the 

 connection of the specimen with some other form, of which it may be but a variety or mere 

 difference in shape. 



Hundreds of British and foreign specimens of Carboniferous Strepiorliynchus have 

 been assembled and carefully examined, but after much research and uncertainty from not 

 finding characters of sufficient permanence to warrant the establishing of distinct species, 

 I resolved (provisionally so, at least) to retain but one, of which S. crenistria 

 (Phillips) may be considered the type, and to describe under separate heads, but with 



1 Darwin ' On the Origin of Species.' 



