Vol. 58.] REFERRED.TO MURCHISONTA AND PLEUROTOMARIA. 337 
Upper Orthoceratite-Limestone of Lerkaka (Oland) for comparison. 
I find that P. girvanense differs in being much smaller, being about 
one third the size; the upper surface of the whorl is flatter and 
almost concave, while Pleurotomaria elliptica is slightly convex; below 
the band the base is more convex, while P71. elliptica is flattened : the 
upper part of the body-whorl also appears longer in proportion ; 
but the only known specimen is slightly flattened by pressure, so it 
cannot be accurately compared in: this particular. ‘The whorls are- 
less angular at the periphery, the three keels forming the band on 
the penultimate whorl are nearly equal in strength, and the lower 
one coincides with the periphery; while in Pl. ellptica the central 
one appears the strongest and most prominent. They agree, how- 
ever, in having the central thread of the band less developed on the 
latter part of the body-whorl. Both species are quite distinct from 
Trochus ellipticus of Portlock,’ which latter is so much crushed that 
it would be difficult to compare other specimens with it accurately. 
Dimensions.—The length=11 millimetres, the width=11 mm., 
and the length of the slit=about 5 mm.: if the lip were entire, it 
might possibly be longer. It equals about one-fifth of the circum- 
ference of the body-whorl. 
Locality and Horizon.—Ardmillan Braes (Ayrshire), in rocks 
of Llandeilo age [Lapworth]. 
Family TURRITELLID 4S, Lam. 
Genus Aclisina, de Kon. 
ACLISINA (?) oBscURA, sp. nov. (Pl. IX. fig. 12.) 
Diagnosis.—Shell small, very elongated, turreted, composed of 
about thirteen whorls. Whorls increasing gradually, flat above, 
slightly convex below. There is a strong keel near the middle of 
the whorl, with two similar keels at equal distances apart below. 
and traces of a fine thread above them, and another thread im- 
mediately below the suture. Base convex, moderately produced. 
Apertures and lines of growth unknown. 
Remarks and Resemblances.—There are but two examples 
of this species known, which are in Mrs. Gray’s collection. As the 
aperture and lines of growth are not preserved on either of them, ” 
it is difficult to ascertain to which genus they should be referred. 
There does not appear to be any trace of a true sinual band, 
therefore they cannot be placed in Murchisonia. The elongated 
form and ornamenting keels resemble those of both Hctomaria and 
Aclisina, and more especially the latter, the keels being finer than 
is usually the case in Ectomaria. It reminds one forcibly of certain 
Carboniferous species of Aclisina; and should it prove really to 
1 “Geol. Rep. Londonderry ’ 1843, p. 414 & pl. xxxi, fig. 1. 
? This genus is fully described in Quart. Journ. Geol. Soc. vol. liv (1898) 
p. 45. 
