872 DR F. R. C. REED ON THE 



two-thirds to three-fourths its length ; there is one strong raised concentric ridge 

 at the geniculation, but on the disc, which is somewhat sunken and depressed, there 

 are only faint or narrow concentric rugae (PI. XII, fig. 32). 



(2) (var. /3.) The specimens from Shalloch Mill have pointed ears; the shell 

 is geniculated at three-fourths to four-fifths its length ; there are 8-1.0 equal and 

 regular concentric rugse on the disc, and they bend out to meet the hinge-line at 

 acute angles. The examples from Whitehouse Bay have the rugae directed more at 

 right angles to the hinge-line (PL XIII, fig. l). 



(3) (var. 7.) The form from the Starfish Bed, where the species is very 

 abundant, has the shell usually auriculated ; the geniculation is sharp and at one- 

 half to two-thirds the length of shell, and is continued all round ; the disc is semi- 

 circular to semielliptical or subquadrate, and carries 8-11 fairly regular strong 

 somewhat wavy concentric rugae increasing in strength to the geniculation, and 

 they mostly meet the hinge-line at right angles, only the outermost 2 or 3 bending 

 out to meet it at acute angles (PL XIII, figs. 2, 3). 



(4) (var. S.) The Mulloch Hill shells are of semicircular shape, with slightly 

 pointed ears ; the geniculation takes place at about two-thirds to three-fourths the 

 length of the shell; there are 8-10 concentric rugae meeting the hinge-line at about 

 75°-80°, but the outer ones do not bend out to meet it more acutely (PL XIII, fig. 4). 



(5) (var. e.) The specimens from Woodland Point tend to be emarginate iu 

 front, with a broad median flattening or shallow sinus in the bent-down portion, which 

 is produced in the middle into a tongue, and the anterior edge of the disc is some- 

 what swollen and raised on each side. The geniculation takes place at about two- 

 thirds to three-fourths the length of the shell, but in the young shells at about 

 one-half the length. The larger shells have well-produced ears, and the concentric 

 rugae on the disc meet the hinge-line at an acute angle, except the innermost 2 or 3 

 (PL XIII, figs. 5-7). 



(6) (var. £.) The Upper Llandovery forms from Penkill and Bargany Pond 

 Burn have long, pointed ears and 12-14 (or more) rather closely placed regular 

 concentric rugae, of which all the outer ones bend out to meet the hinge-line at 

 acute angles. There is also a median group of 6-8 noticeably stronger radii on the 

 surface, with 2-4 smaller ones between each pair, though over the rest of the surface 

 there is no distinction into stronger and weaker radii (PL XIII, figs. 8, 9). 



Horizons. — Var. a (l) Stinchar Limestone Group ; (2) Balclatchie Group; var. ft 

 (3) Whitehouse Group ; var. y (4) Drummuck Group (Starfish Bed) ; var. 8 (5) 

 Mulloch Hill Group ; var. e (6) Saugh Hill Group ; var. £ (7) Camregan Group ; 

 (8) Penkill Group. 



Localities. — Var. a (l) Craighead, Minuntion ; (2) Balclatchie (conglom.) ; var. P 

 (3) Shalloch Mill, Whitehouse Bay; var. y (4) Thraive Glen; var. S (5) Mulloch 

 Hill, Craigens ; var. e (G) Woodland Point, Newlands ; var. £ (7) Bargany Pond 

 Burn ; (8) Penkill, Penwhapple Glen. 



