32 BRITISH CARBONIFEROUS BRACHIOPODA. 



tional individuals of two or three approximate species. It is, however, distinguished from 

 Sp. trigonalis, with which it has been often confounded, by the characters of its mesial 

 fold, which does not project much beyond the level of the lateral margins, and is also 

 regularly rounded, and not acutely elevated as in Martin's shell. 



Professor M'Coy considers the form under description to be a simple variety of Sp. 

 trigonalis, and states that " the most distinctive character of this variety {bisulcata), 

 besides its gibbosity, is the abrupt rising of the mesial sinus at the margin, nearly at right 

 angles to the plane of lateral edges, shortening the length of the middle of the ridge, 

 notching the front margin, and giving a nearly semicircular curve to the profile of the 

 receiving valve," &c. 



It is distinguished from Sp. mosquensis and striata by the simplicity of its ribs, and 

 from the Devonian Sp. aperturata, to which it has been assimilated, by the greater depth 

 of the sinus, as well as the elevation of the fold of Schlotheim's species. Sowerby's type, 

 which I have illustrated from the original specimen (PI . VI, figs. 6 to 9 ), is very gibbous, 

 and much less transverse than is the case with a vast number of individuals of the species, 

 which also present wing-shaped expansions of greater or lesser extension, as may be seen 

 from the series of examples which have been selected for illustration. It seems rather 

 surprising that Sowerby did not perceive that the specimens, tab. 565, figs. 2 and 3 

 (13 and 14 of our plate), did belong to his species, and not to Sp. trigonalis to which 

 he refers them, a mistake subsequently copied by various authors. 



Sp. semicircularis, Phillips, does not seem to possess any characters distinguishable 

 from Sp. bisulcata, and I have therefore followed Professor De Koninck in adding the 

 name to the synonyms of the species under description. 



The most remarkable peculiarity in Sp. bisulcata is the tendency of the ribs on the 

 mesial fold to divide into three distinct groups, separated by sulci of greater depth ; and 

 although this may be also observed at times in other Spirifers, such as on some young 

 examples of Sp. striata, still it is nowhere better exemplified than in the shell under 

 description. Sp. bisulcata appears to have attained much larger dimensions than any 

 example of Sp. trigonalis I have hitherto examined. 



The shell figured as Sp. calcarata by Professor M'Coy, in the ' Synopsis,' pi. xxi, fig. 3 

 (PL VII, fig. 4, of our plates), does not belong to the species so named by Sowerby : it is 

 probably a malformation of Sp. bisulcata. The fine specimen, PI. IV, fig. 1, seems like- 

 wise referable to one of the forms of Sowerby's variable species, and Sp. transiens, M'Coy, 

 may also, perhaps, be an allied form ; but as the original specimen is no longer to be 

 found in Dr. Griffith's collection, and knowing of no other individual, there may exist 

 some doubts as to the correctness of this last supposition. 



Loc. Sowerby's original specimens are said to be from Dublin and Derbyshire ; it 

 occurs abundantly in several localities of the last-named county, such as at Dovedale ; 

 is also common in the Wensleydale and Yorkshire districts, at Lowick, Northumberland, 

 in the Isle of Man, &c. 



