30 BRITISH CARBONIFEROUS BRACHIOPODA. 



In addition to these, the surface of each valve is ornamented by from twenty to twenty- 

 two simple ribs. The dimensions, taken from two examples, measured — 

 Length 11, width 13^, depth 8 lines. 

 „ 15, „ 18, „ 10 lines. 

 Obs. Martin states that " the general form or outline of this shell is trigonal or three- 

 cornered, with the angles rounded off (our fig. 25) ; its surface longitudinally furrowed 

 and sinuated ; the furrows rounded, their number varying from twenty to thirty ; the 

 sinus continued, rounded, and extending the breadth of three or four furrows, hence 

 striated like the other parts of the surface," &c. The species was not sufficiently illus- 

 trated, which has led subsequent authors to combine with it other allied forms, so as to 

 obscure its distinctive characters. Thus, for example, in tab. 265 of the 'Mineral Con- 

 chology,' fig. 1 alone would belong to Martin's shell, while figs. 2, 3, and 4 are referable 

 to Sowerby's own Sp. bisulcata, a fact I was able to confirm from the close examination 

 of type examples of each species. 1 I am quite ready to admit Sp. bisulcata to be a 

 nearly allied form, but am not yet entirely convinced that Professor M'Coy is right 

 while considering Sovverby's shell as a simple variety of that of Martin's. Both may be 

 distinguished by differences in general shape, the mesial fold in Sp. trigonalis being much 

 more elevated, angular, prolonged, and possessing fewer ribs than in Sp. bisulcata, where 

 the fold is much flatter, or more regularly rounded. Its hinge-line is also more extended, 

 and the contour of the shell more semicircular and less trigonal than in Martin's shell* 

 Nor can I agree with Professor M'Coy when he places Sp. rhomboidea, Phillips, among 

 the varieties of the species under description. 



Some young and exceptional examples of Sp. triangularis approach much to Sp. 

 trigonalis, but in full-grown individuals the differences are well defined. The illustration 

 attributed to Sp. trigonalis by Professor De Koninck, in his work on the 'Animaux 

 fossiles de la Belgique,' pi. xvii, fig. 1, does not convey an adequate idea of Martin's 

 species, to which in all probability it does not belong. I agree with Professor M'Coy 

 that Eichwald's Sp. incrassata, as figured in the ' Geol. of Russia/ pi. vi, fig. 3, may be 

 referred to Sp. trigonalis, but De Verneuil's Sp. Strangwaysi, fig. 1 of the same plate, is 

 not equally certain. 



Loc. This species is very abundant in the lower Carboniferous limestone of many 

 British localities. Martin's type was derived from Derbyshire. Mr. Tate has it from 

 Denwick and several other localities in Northumberland. It is found at Buxton. At 

 Bakewell, in Derbyshire, several beautiful examples were obtained by Mr. Binney, from a 



1 Professor M'Coy seems to have hesitated to admit as Sp. trigonalis the above-named figures pub- 

 lished by Sowerby, for we find stated, at p. 424 of the work on the 'British Palaeozoic Fossils' — "I have 

 not quoted Sowerby's figures ('Min. Con.,' tab. 265), as he gives no definition to the mesial ridge, and 

 makes the ribs so much broader than any of the great number of specimens I have examined, that it is 

 scarcely recognisable as a portrait of the ordinary forms." Professor M'Coy's description of Martin's 

 shell is both detailed and correct. 



