AVICULOPECTKX SEMICOSTATUS. 69 



Aviculopecten semicostatus, Portloch, sp., 1843. Plate XIII, figs. 9 — 15. 



Pecten i'Licatus, Phillips, 1830. Geol. Yorks., pt. ii, p. 212, pi. \i, fig. 21. 



— semicostatus, Portloch, 1843. Rep. Geol. Londonderry, p. 436, pi. xxxvi. 



fig. 9. 

 flexuosus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 93, pi. xviii, 

 fig. 1. 

 Aviculopecten docens, M'Coy, 1855. Brit. Pal. Foss., p. 485, pi. 3 e, 11l;'.s, t\, 7. 

 illegalis, M'Coy, 1885. Ibid., p. 486. 



Specific Characters.- — Shell of medium size, the left valve moderately convex, 

 the right much less so, nearly orbicular. Ears strongly defined, the anterior 

 depressed, a little shorter than the posterior, falcate, and pointed. The anterior 

 ear in the right valve small, rolled, and separated from the rest of the valve by a 

 deep, broad depression for the byssus. The umbo of the right valve markedly 

 triangular, pointed, and not raised, that of the left valve more gibbose, pointed, 

 and slightly raised. 



Interior. — The adductor muscle-scar is large and shallow, placed high up and 

 just posterior to the middle line. The hinge-plate is broadly diamond-shaped, 

 obscurely striated longitudinally. The internal surface is smooth in the umbonal 

 region, but there are gradual indications of the radiating ribs, which become 

 strongly marked near the margin. 



Exterior. — The surface is ornamented with numerous simple, somewhat 

 irregular, rounded, radiating ribs. Secondary ones often arising in the spaces 

 between the primary, and continuing to the margin. The ribs arc crossed by 

 occasional concentric lines of growth. The posterior ear is crossed by markedly 

 strong concentric folds parallel with the margin, and lias three or four radiating 

 ribs very lightly marked or almost obsolete. The anterior ear has both radiating 

 and concentric strhe, the former faintly seen on the internal east. 



Dimensions. — -PI. XIII, fig. l .», the type specimen, measures — 



Antero-posteriorly . . . . 40 mm. 



Dorso-ventrally . . . . 38 nun. 



Localities. — England: the Carboniferous Limestone of Lowick, and the Redes- 

 dale Ironstone, Northumberland ; the Carboniferous Limestone of Castleton, 

 Derbyshire, and Poolvash, Isle of .Man. Ireland : the Carboniferous Limestone of 

 Ballygasey, Loughgall, co. Armagh; Carnteel, co. Tyrone; St. Doulagh's, 

 co. Dublin. 



Observations. — The type of Portlock's Pecten semicostatus is the cast of the 

 interior of a left valve, preserved in the Museum of the Geological Survey, Jermyn 



