216 MR F. R. COWPER REED ON 



Pterotheca drummuckensis, sp. nov. (PI. III. figs. 4, 5.) 



Shell subcircular to transversely subcordiform, rather broader than long. Apertural 

 margin emarginate, with broad, rounded lateral lobes, and broad, angular median notch 

 hot ween them. Median fold narrow, sharp, acutely angular, not projecting on apertural 

 margin, but corresponding with angular notch. Lateral folds very low, wide and inde- 

 finite, passing into flattened lateral portions of shell. Posterior margins of shell meeting 

 at apex at about 100°-110°. Pair of narrow shallow grooves diverging forwards from 

 apex at about 45°, halfway between median ridge and lateral margins of shell. 



Surface marked with regular strong corrugations, concentric to apertural margin, and 

 bending back sharply in median notch so as to form re-entrant angle of about 50° on 

 ridge of keel. 



Dimensions : — 



I. II. 



Length ...... 20-0 150 mm. 



Width ...... 20-0 18-0 „ 



Horizon and Locality. — Drummuck Group, Thraive Glen. 



Remarks. — This shell, though only known by imperfect specimens, differs from the 

 next described species, Pt. girvanensis, by its shape, its non-projecting median keel, the 

 angle of divergence of the grooves on the surface, the acutely emarginate apertural 

 margin, and the regular corrugation. 



Pterotheca girvanensis, sp. nov. (PI. III. figs. 6-13.) 



Shell transversely subelliptical, widest towards the front. Apertural margin sinuous, 

 projecting in middle in short subangular tongue, excavated slightly on each side, and 

 with rounded, more or less projecting lateral lobes, forming the angles and merging into 

 the lateral margins, which are rounded. Apex small, sharp, incurved. Surface of shell 

 gently convex, elevated into sharp median longitudinal fold, proceeding from apex with 

 increasing height, width, and acuteness to the apertural margin, where it stands up 

 prominently and projects as the median tongue ; a pair of broader but low, rounded, 

 indefinite lateral folds run forwards from the apex divergently, almost dying out on the 

 apertural margin, where they form the lateral lobes. Two shallow definite grooves, 

 slightly curving outwards, diverge from the apex at about 60°-70°, and run forwards 

 towards the outer lateral angles of the apertural margin, nearly or quite reaching it, and 

 they lie on the outer slopes of the lateral folds. These grooves correspond internally 

 with low ridges or septa. Internal concave plate present, extending about two-thirds 

 the length of the shell. Surface of shell minutely punctate, and with more or less de- 

 veloped concentric corrugations and strise following the outline of the apertural margin. 



Dimensions : — 



I. IT. III. 



Length ..... 15-0 90 13"0 mm 



Width ..... 20-0 12-5 20"0 „ 



