236 CARBONIFEROUS LAMELLIBRANCHIATA. 



Dimensions — 



Antero-posteriorly. Dorso-veutrally. Laterally. 



PI. XIX, fig. 8, from Easter Bucklyvie, Donibristle 18 mm. 15 mm. — 



PI. XIX, fig. 7, from same locality . . 28 mm. 24 mm. 5 mm. 



Localities. — Scotland : Ayrshire, Beitli ; in shale below the Main Limestone, 

 Lower Limestone series ; "Woodtop Quarry, Teasses ; Easter Bucklyvie, Doni- 

 bristle ; Pathhead, Lambland Quarry ; Encrinite bed, Broom Hill, St. Andrews. 

 Ireland : in Arenaceous shale at Mullaghtenny, Clogher, co. Tyrone. 



Observations. — I find this species in some Scotch cabinets under the name 

 " Axinus axiniformis, Phillips," but it differs from P. axiniformis in its obliquity 

 and by the much more strongly pronounced diagonal ridge. It has also a much 

 shorter anterior end and hinge-line, and the umbones are less twisted forwards. 

 I refigure the type specimen of M'Coy's Axinus obliquus, PI. XIX, fig. 9, which 

 is preserved in the Griffith Collection at the Museum of Science and Art, Dublin. 

 This is a left valve, and not, as represented in the original drawing, a right valve. 



M'Coy's description states that the shell possesses " a strong ridge from the 

 beak to the posterior angle," and this is well shown in Scotch specimens. The 

 following statement also occurs — " Epidermis produced into long fringes beyond 

 the margin," but on this point I am not able to offer an opinion, as I have not 

 been able to observe this character in any specimen ; and the members of this 

 genus do not, as far as I know, possess a periostracum of any thickness. I 

 figure several testiferous examples from the collection of the Geological Survey, 

 Edinburgh. 



This species resembles P. axiniformis more nearly than any other, but is dis- 

 tinguished from it by its greater degree of obliquity, the very rapid descent of the 

 posterior border, the marked angulation of the oblique ridge, and the very narrow 

 adpressed dorsal slope. It is also less convex. P. obliquus is found at about the 

 same horizon as P. axiniformis in Scotland, but, as far as I know, does not occur 

 in the Upper Limestone series. In Ireland the species seems to be confined to the 

 Lower Limestone shales, an horizon which I believe, on strong palaeontological 

 grounds, to be the equivalent of the Calciferous-sandstone series of Scotland. 



M'Coy {op. cit.) redescribed his species, taking for a second type some shells 

 from the Limestones of Lowick, Northumberland ; but I have never seen this 

 species from that locality, and the fine series of shells in the Woodwardian 

 Museum, Cambridge, have nothing in common with the shell which was the 

 original type. The description, however, agrees in every particular with the 

 original specimen, and with those which I now refigure as typical of the 

 genus, except perhaps that it makes the shell too convex. I am of opinion that 

 specimens of P. axiniformis, which are fairly common at Lowick, must have been 

 mistaken for P. obliquus. 



