Vol. 67.] ASSOCIATED KOCKS OF NORTH-EAST MONTGOMERYSHIRE. 451 



that of the pleurae. The lobes are separated by distinct furrows, 

 which broaden towards the margin of the tail. 



Remarks. — The species is somewhat like Ampyx nudus, 1 but 

 the shape and character of the tail distinguish it from that species. 

 It bears a rather close resemblance also to Dionide ricliardsoni 2 ; but 

 its broad shape, the narrow flattened tail, and the grooves in the 

 cheeks are all points of difference. I believe the form to be a new 

 species ; owing, however, to the imperfect state of the head, I have 

 hesitated to name it. 



Horizon and locality. — Trilobite-Dingle Shales, Middle 

 House Dingle, Guilsfield. 



(2) Ostracoda. 

 Genus Melanella, nov. **f, Jj^^M*^ *M«*X 





Jonesella (pars) Ulrich, Journ. Cine. Soc. Nat. Hist. vol. xiii (1890-91) p. 121. 



Carapace small, equivalve, moderately convex, subcircular, rather 

 flattened anteriorly ; hinge straight ; valves with a small, faintly- 

 raised horseshoe ridge confined to the posterior half, enclosing a 

 small semicircular sulcus. Edges simple. 



This genns is closely allied to Jonesella of Ulrich, and less 

 intimately to Bollia of Jones & Holl. 3 In the latter genus the 

 horseshoe ridge is larger and more central, while the edges of the 

 valves are thickened. In Jonesella the loop is a well-developed 

 ridge, which is usually confined to the posterior half or two-thirds, 

 but it occupies a greater portion of the valve than it does in 

 Melanella. I have reproduced Ulrich's figure of Jonesella crepidi- 

 Jormis for comparison (see fig. 9 c, p. 452). Jonesella obscura Ulrich 

 is most closely allied to the present form, and should be placed in 

 the new genus. Ulrich, in describing this species, 4 shows plainly 

 the need for a new genus here. He calls J. obscura ' an incipient 

 Jonesella,' and shows that it has characters which are allied both to 

 Bollia and to Ulrichia. 



Melanella hemidiscus, gen. et sp. nov. (Figs. 9 a & 9 b, p. 452.) 



Length =0*76 millimetre; height =0*65 mm. 



Valves moderately convex, almost semicircular in outline. Hinge 

 straight and rather short, slightly rounded at the ends. Horse- 

 shoe ridge low and obscure, entirely within the post-dorsal half, 

 enclosing a sulcus which contains a low tubercle. The species 

 differs from Melanella (Jonesella) obscura in the more semicircular 



1 E. Forbes, ' Brit. Organic Rem.' Mem. Geol. Surv. dec. ii (1849) pi. x. 



2 F. R. C. Reed, ' Lower Palaeozoic Trilobites of the Girvan District, Ayr- 

 shire' Monogr. Pal. Soc. (1903-1906) p. 26 & pi. iv, figs. 3-8. 



3 T. R. Jones, ' On some Devonian & Silurian Ostracoda from North 

 America ' Q. J. G. S. vol. xlvi (1890) pp. 540, 548 & pis. xx-xxi. 



4 'Geology of Minnesota' Geol. Nat. Hist. Surv. Minn. vol. iii, pt. 2 (1897) 

 pp. 667, 668. 



