part 2] SYHINGOTHYHIS AND SPIRIFEBINA. 215 



tipiriferinu cf. octoplicata ; while Vaughan compared certain of 

 them with the Belgian species, Sp. peracnta De Koninck. 



Since the three species mentioned are most typically developed 

 in the Dibunophyllum Zone, and the holotypes of the species 

 already described (namely, octoplicata and insculpta) were 

 obtained from beds of that age, it seems desirable to select a form 

 from that horizon as the holotype of the new species perplicata, 

 and to deal first with the three species as typically developed,. 

 before discussing their earlier mutations. 



SiMRiFERiNA octoplicata (J. de C. Sowerby). (Text-tigs. 4 q & 

 5 b, pp. 199, 211 ; PI. XIII, figs. S & 9.) 



1827. Spirifer dctopHcatus J. de C. Sowerby [32] p. 120, & pi. dlxii, tigs. 2-3, 



nou tig. 4. 

 1844. Spirifer cristatus M'Coy [20J p. 133. 

 1859. Spiriferina cristata, var. octoplicata Davidson [4] p. 38 & pi. vii, figs. 37- 



47 (probably only tigs. 38-41 really belong to this species). 

 1915. Spiriferina octoplicata (Sowerby), Vaughan T35J pi. iii. 



The original description is as follows :— 



' Transversely elongated, gibbose, semicircular, plaited ; plaits 8 or 10, deep 

 and angular ; central elevation plain ; beaks remote, incurved ; area tri- 

 angular, curved.' 



Description. — Shell small, transverse, cardinal extremities 

 slightly rounded. The dimensions of a specimen of average size 

 are : width along hinge-line, 20 mm. ; length, 10 mm. ; thickness, 

 8 mm. The area of the pedicle-valve is concave, the curvature 

 increasing towards the beak ; it is defined by a pair of lines 

 diverging from the apex, and gradually curves over into the lateral 

 slopes. The beak of the pedicle-valve is small, pointed and in- 

 curved, and that of the brachial valve is tumid and incurved. 



Both valves are markedly and almost equally convex. The 

 mesial fold is angular, but is frequently flattened anteriorly ; the 

 mesial sinus is also angular, and is sharply defined by a pair of 

 angular costae originating at the beak. The lateral slopes of each 

 valve are ornamented by about four angular costae : those on either 

 side of the median fold are rather more than half as wide as the 

 fold, and the remainder are progressively smaller. The costse are 

 separated by dee]) angular furrows. Of the costse in the brachial 

 valve, only three on each side of the fold originate at the beak, 

 so that in small immature specimens each lateral slope has but 

 three costae. Median septum in the pedicle-valve attached to the 

 floor of the valve for about a third of its width, thick at the 

 base, it rapidly tapers and becomes knife-like ; near the beak it 

 rises about half-way to the level of the area, and becomes gradually 

 lower anteriorly. Its posterior portion is embedded in a slightly- 

 developed apical callosity- 

 Surface of the shell marked by a few concentric lamella? which, 

 like those in Sp. walcotti, are widely spaced, and occur chiefly in 

 the anterior region. The flaking-away of the external Layers often 

 leaves the surface of the shell smooth, and is responsible for the 

 characteristic appearance of main' of the Derbyshire specimens. 

 (\. .!.(J.S. No. 802. Q 



