362 Prof. T. R. Jones and Dr. H. B. HoU on tie 



deep median sulcus separates the two limbs of this thick, 

 curved, lobular ridge, and the surface of the valve sinks deeply 

 all round outside the lobes, between them and the narrow 

 ridge which stands just within the fore and aft margins. 

 This submarginal ridge represents the thicker semilunar 

 ridges in B. hicollina. 



The surface has a reticulate ornament. The dorsal aspect 

 of the carapace is irregularly subovate, blunt at the end, and 

 lumpy at four spots with the ends of the lobes. 



At first sight this appears to be possibly a varietal form of 

 the foregoing (p. 361, figs. 14, 15, and 16), but we find no 

 intermediate stages. 



This species is evidently an ally of Bollia cohoaUensis 

 (Beyrichia), Holl (Geol. Mag. dec. ii. vol. viii. 1881, p. 346, 

 pi. X. fig. 14) ; and in the simply curved elevation of B. 

 comma, Jones (Sil. Foss. Girvan, 1880, p. 219, pi. xv. fig. 9), 

 perhaps we have a distant homologue of the great curved lobe, 

 consituting the characteristic feature both in the unique speci- 

 men before us and in B. hicollina. These two are closely 

 related ; they may be only sexually different, but convenience 

 callsfor distinctive names, since a specific standing is possible. 

 The Lower-Silurian B. Grewingkii, Bock (Neues Jahrb, &c. 

 1867, p. 594), according to the description, may also be an 

 ally. 



The broader lobes and thinner semilunar ridges, together 

 with the semicircular outline of the valves, distinguish B. 

 unijiexa from B. hicollina. The dorsal outlines are easily 

 recognized on account of the lobes forming more isolated pro- 

 jections in the latter species. 



Vine Coll. no. xxxviiii (only one specimen, with B. 

 Kloedeni, var. tvherculata, Salter), from bed no. 25. 



III. Klcedenia, gen. nov. (See above p. 347.) 



1. Kladenia intermedia, Jones & Holl, var. marginata. 

 (PI. XII. figs. 21a, 5.) 



Proportions :— L. 9. H. 6. Th. 5. 



Beyrichia interinedia, J. & H., Ann. & Mao^. Nat. Hist. ser. 4, vol. iii. 

 1869, p. 218, pi. XV. fig. 7 J Jones & Kirkby, ibid. vol. xv. p. 55, 

 pi. xvii. fig. 11. 



This neat little form, one specimen of which has been col- 

 lected by Mr. Vine (No. LIV12, hed no. 37) agrees with the 

 species referred to above, except that it has a marginal rim, 

 and does not show the little foremost furrow. 



It evidently belongs to the same group as K. Wilckensiana, 

 namely, what were called the Beyrichioi corrugatce ; and tlie 

 smooth convexity of the main portion of the valve, the very 



