816 DR F. R. C. REED ON THE 



posterior margin is wanting, and this seems to be an essential character of Acrotreta 

 Nicholsoni. In the brachial valve of Acrotreta, Walcott (op. cit.) mentions a long, 

 well-defined median ridge and a pair of strong cardinal tubercles near the margin of 

 the area, but these cannot be clearly detected in any of the Girvan specimens. 



Some of Davidson's figured specimens of A. Nicholsoni from Girvan have been 

 removed to Micromitra Davidsoni (p. 797), and others must be placed in a new species 

 of Conotreta (C. conoidea, q.v.) ; while others labelled A. Nicholsoni by Davidson 

 seem to belong to the genus Acrothelc rather than Acrotreta. Consequently it is highly 

 doubtful if the Moffat form A. Nicholsoni really occurs in the Girvan area at all. 



Acrotreta (?) sp. a. 

 (Plate III, fig. 28.) 



There are some small calcareo-corneous brachial valves occurring in the Stinchar 

 Limestone and Balclatchie Groups which are probably referable to some species of 

 this genus, and somewhat resemble Acrotreta curvata, Walcott, # from the passage 

 beds between the Cambrian and Ordovician of Eureka, Nevada. The valve is sub- 

 circular in shape ; the beak is small, acute, and marginal, and from it a shallow 

 depression may run forward across the flattened surface of the valve to the front 

 edge. The ornamentation consists of fine concentric lines. The cardinal scars can 

 be rarely detected. 



Dimensions. — Diameter about 2'0 mm. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead; (2) Balclatchie. 



Acrotreta (?) sp. b. 

 (Plate III, figs. 29-31.) 



Shell transversely subcircular. Brachial valve gently convex, with small pointed 

 marginal beak ; interior with long straight septum extending about three-fourths the 

 length of valve ; pair of small triangular cardinal scars contiguous to septum in median 

 line close to beak, and a pair of similarly placed small subcircular " central" (?) scars 

 at about one-third length of valve. External surface of shell with concentric growth- 

 lines, and a few fine radial striae in interior. 



Dimensions. — Length, c. 4 mm. ; width, c. 5 mm. 



Horizon. — Stinchar Limestone Group. 



Localities. — Bennane Burn, E. Stinchar ; Craighead. 



Rem,arks. — One specimen of the interior of a brachial valve [2809, Geol. Surv. 

 Mus., Edinburgh] shows the muscle-scars and internal septum which suggest its 

 reference to the genus Acrotreta. The others merely show the internal septum. 



* Walcott, op. cit., p. 682, pi. lxviii, figs. l/-li. 



