LOWER PALEOZOIC HYOLITHID^ FROM GIRVAN. 217 



Horizon and Localities. — Balclatchie Group, Balclatchie and Ardmillan. 



Remarks. — This species is undoubtedly closely allied to Pt. corrugata, Salter. 

 The median slit which the latter species is represented as possessing near the anterior 

 margin is apparently due merely to the breaking off of the acute carina, as precisely 

 the same appearance is met with in damaged specimens of this Girvan form. There are 

 three lateral folds represented by Salter on each side of the central one, and the con- 

 centric strise are said to meet the keel abruptly at a right angle, whereas in ours they 

 pass over it continuously, following the outline of the apertural margin. 



Pterotheca mullochensis, sp. nov. (PL III. figs. 18-20.) 



Shell transversly elliptical to subcircular. Apertural margin arched forwards, not 

 emarginate, with small projecting triangular tongue in middle. Surface gently convex. 

 Apex small, incurved. Median fold angulated, acute, narrow and weak near beak, but 

 increasing in height and width to front margin, where it projects slightly as tongue. 

 Lateral lobes on apertural margin and lateral folds on surface obsolete, but halfway 

 between median fold and lateral margin of shell the surface is angulated, being bent 

 down rather suddenly on each side, along lines diverging from the apex at about 60° ; 

 the angulations die out towards the anterior margin, curving gently outwards, or 

 running nearly straight forwards ; they correspond near their origin to the course of the 

 thin internal divergent septa which start from the apex and extend about one-third to 

 one-half the length of the shell. Internal plate gently concave, extending forwards 

 rather more than half the length of shell. Surface marked with numerous moderately 

 strong concentric corrugations and stria?. 



Dimensions : — 



I. II. III. 



Length ..... 24-0 22-5 23-0 mm. 



Width ..... 26-0 27-0 c. 25-0 „ 



Horizon and Locality. — Mulloch Hill Group, Mulloch Hill ; and (?) Saugh Hill 

 Group, Newlands. 



Remarks. — The shape of the anterior margin, the obsolescence of the lateral lobes 

 and folds, the angulation of the lateral surfaces and general shape of the shell, dis- 

 tinguish it from Pt. girvanensis and Pt. drummuckensis. Portlock's Pt. transversa 

 from Desertcreat, the type of which is in the Jermyn Street Museum, much resembles 

 Pt. mullochensis in surface characters, but is broader and more transversely elliptical in 

 shape, its length to its width being as 2-3 ; and it possesses, near the beak, a small 

 additional pair of very short septa between the two long divergent ones, and the latter 

 bend away at rather a wider angle, though this may be due to crushing. The con- 

 centric rugse are less developed, the concentric markings being rather of the nature of 

 fine wrinkles and striae. 



