122 BRITISH CARBONIFEROUS BRACHIOPODA. 



recognisable as a distinct variety much in the same way as the Devonian variety aspera, 

 of the Silurian Spirigerina reticularis, is usually distinguishable from the older varieties. 

 The general characters of the species are exactly those of Lept. depressa, and the 

 description given in the Lower Palaeozoic section of this species will suit the present one in 

 nearly all respects. The differences which strike an observer are, that the transverse 

 wrinkles in the Lower Palaeozoic L. depressa are usually from twelve to fifteen in number, 

 but in this variety they are from fifteen to twenty, and the longitudinal striae are, on an 

 average, considerably coarser in L. analog a than in L. depressa." The last two 

 observations are not, however, of much importance; for I possess examples of the Silurian 

 and Devonian shell with as many wrinkles as may be counted upon the generality of 

 Carboniferous specimens, nor does there exist that great difference in the quality of the 

 longitudinal striae. Professor M'Coy then goes on to observe, that as to the varieties 

 named Lept. distorta by Sowerby (Carboniferous), and Lept. nodulosa by Phillips (Devonian), 

 he has in the mountain limestone traced all the intermediate varieties to the more regular 

 types, and no doubt exists in his mind as to their specific identity. It is therefore 

 evident that, in external shape and character, it is hardly possible to distinguish the 

 Silurian, Devonian, and Carboniferous shell, and that those little differences that are 

 occasionally observable, such as number of wrinkles and coarseness of longitudinal striae, 

 may be due to local conditions which often slightly modify the size and appearance of the 

 same species. 



Now let us cast a glance at the interior of the Silurian and Carboniferous shell 

 In the interior of the valves no very important differences can be traced, and the only 

 peculiarity I can perceive is, that the ridges margining the muscular scars are generally 

 more elevated or produced (though not always so) in the Carboniferous specimens ; and it 

 appears to me, therefore, that by retaining the term analoga as a varietal designation 

 for the Carboniferous form, we shall be following the views advocated by the greater 

 number of palaeontologists. 



The variety distorta, which has prevailed in certain localities, can be viewed in no 

 other light but as that of a malformation, in which the dorsal valve is generally more or less 

 convex, and bending more or less rapidly inwards before becoming again inflected to 

 follow closely the deflected prolongation of the ventral valve. 



Loc. In England, the Carboniferous variety is common in the Lower Carboniferous 

 limestone of Longnor, as well as in other Derbyshire localities : it has been collected 

 at Bolland and the Craven and Wensleydale districts, in Yorkshire ; in Carboniferous shales 

 at Redesdale, Northumberland ; also in the Middle Carboniferous limestone of Ronalds- 

 way and Poolwash, Isle of Man. 



In Scotland, it is found at Gare, in Lanarkshire, in limestone, at 239 fathoms below 

 the "Ell Coal," and 343 at Waygateshaw ; in Renfrewshire, near Thornliebank ; in 

 Stirlingshire, in the Campsie Main limestone and Corrie Burn beds. 



In Ireland, it appears to occur in all the Lower Carboniferous strata, as well as in the 



