ORDOVICTAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 917 



equidistant thick raised rounded concentric lines ; lateral margins of sinus marked 

 by one fine strong straight rib, with traces of 2-3 very faint smaller ribs on lateral 

 slopes of valve. 



Dimensions. — Length, 3'25 mm. ; width, 6'00 mm. ; depth at beak, 4'00 mm. 



Horizon. — Whitehouse Group. 



Locality. — Shalloch Mill. 



Remarks. — One of the two small specimens of this peculiar little shell from 

 which the above description is drawn up was found mounted on the same tablet with 

 Davidson's types of CI. shallochensis described below. It is certainly quite distinct 

 from that species, and is closely similar to CI. [Orthisina] complectens, Wiman, # 

 from the Leptsena Limestone of Sweden. From the latter it differs only in having 

 the radial lines scarcely developed at all, and the cancellation of the surface therefore 

 not so noticeable. 



Clitambonites shallochensis (Davidson, emend.). 

 (Plate XXI, figs. 21-23.) 



1883. Skenidium Shallockiense, Davidson, Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 174, pi. xi, 

 figs. 6, 7. 



Shell transversely semielliptical to semicircular ; hinge-line straight, equal to 



maximum width of shell ; cardinal angles usually less than a right angle and more 



or less acute and subalate. Pedicle-valve very deep, conical, with a broad rounded 



shallow median sinus ; beak forming apex of cone, small, pointed, not incurved ; 



hinge-area very large, triangular, at right angles to plane of valve, with large wide 



triangular delthyrium covered in lower part by convex plate leaving large oval foramen 



near beak. Brachial valve very gently convex, more or less flattened, with median 



sinus bilobing it ; beak small, inconspicuous, not elevated ; hinge-area very narrow, 



with broad triangular delthyrium. Surface of valves covered with 40-50 low rounded 



closely placed subequal ribs curving back slightly near cardinal line, and sometimes 



with shorter rather smaller ribs intercalated at about one-fourth their length, all 



crossed by close concentric rather coarse striae. 



Dimensions. — 



(1) (Davidson's fig. 6.) (2) (Davidson's fig. 7.) (3) 



Length 7"0 475 10'5 mm. 



Width 13*0 8*00 17*5 mm. 



Depth of pedicle-valve at beak . 6'5 6'00 9"0 mm. 



Horizon. — Whitehouse Group. 



Locality. — Shalloch Mill. 



Remarks. — Davidson's description was so inadequate that a fresh diagnosis of the 



species based on the type and on the other specimens now available is necessary. He 



was only acquainted with three pedicle-valves, so that his description was unavoidably 



* Wiman, Arkivf. Zool. (Stockholm), Bd. iii, No. 24, 1907, p. 11, t. i, figs. 1-4. 



