Pahcozoic JJivalved Entomostraca. 405^ 



The typical ridging lieve seems to be (within tlie siipvamav- 

 ginal outstanding ridge) an inner, irregularly concentric, and 

 an innermost, variable, three-limbed ridge, like a curved 



trident or a distorted CO; ^1^ t^^i^^ ^^^ some trenchant. A 

 lobular swelling rises within the arms of the tripartite ridge in 

 fig. 7. In all cases the surface is coarsely reticulate. In 

 fig. 8 the supramarginal ridge is so prominent as to be very 

 evident at the sides in the dorsal view (fig. 8 h) ; m fig. 9 it 

 stands out in the ventral view (9 c), but not in the dorsa^ 

 aspect (9 b). 



Figs. 5 and 15 are interiors of such valves as fig. 7. The 

 reticulate ornament is partly visible through the test in these 

 specimens. 



This species occurs in j\Ir. Smitli's collection — no. Hi, 2,5, 

 and no. 70, Lincoln Hill, Ironbridge ; and in Mr. Vine's 

 collection, XLix, bed no. 25. 



The late J. G. O. Linnarsson described and figured an 

 analogous form as Beyrichia costata from the Deyrichia- 

 limestone of Angelin's stage " Regio Trinucleorum D a (?) " 

 of the Lower Silurian (or Cainbiian) of West Gothland. Sea 

 Kongl. Svenska Vetenskaps-Akad. Handlingar, vol. viii. 

 1869, p. 85, pi. ii. fig. 67. 



3. Strepida heyvichioides^ sp. nov. 

 (PI. XIII. figs. 2 and 3.) 



^ ,. rFig. 2: L. 9. H 



Proportions: 1 3,j|_ 3. ^^^^ ^ 



2: L. 9. H. 5. 



10. 



In fig. 2 we have a small, neat, almost semicircular valve^ 

 bearing two concentric ridges — one outstanding, sharp, and 

 above and parallel with the ventral margin, which it hides ; 

 the other less regular and not quite entire, within the former. 

 An oval isolated lobe occupies the centre of the valve. 



This form comes near to Strepida concentrica on one hand, 

 and on the other to some of the varieties of Beyrichia Kkedeni, 

 especially as one end of the inner ridge is sublobular^ faintly 

 imitating the giyot-lohe. The outstanding supramarginal 

 ridge, the almost continuous inner ridge formed by the two 

 end lobes, and the isolation of the central lobe, taken alto- 

 gether, give it a peculiar character. 



Fig. 3 shows a larger form, somewhat similar in outline 

 and general aspect to fig. 2. Here the outer or supramarginal 

 ridge is strongly prominent, and two asymmetrical, obliquely 

 transverse, narrow, rough lobes almost meet below an isolated, 

 guttiform, subcentral lobe. 

 A7171. & Mag. N. Hist. Ser. 5. Vol. xvii. 28 



