116 DEVONIAN FAUNA. 



situated at the anterior seventh of the length. Contour gently convex, becoming 

 transversely flat or slightly concave on the back, and sometimes with a slight 

 post-umbonal ridge which vanishes before reaching the postero-inferior corner. 

 Surface with thirty or forty flat, step-like, regular ridges, possibly lamellar, narrow 

 and divaricating in front, becoming gradually larger behind, and indistinct and 

 confused in the postero-superior region, which seems bounded above by a rounded 

 ridge or convexity close to the margin. Cast marked on and near the umbo 

 with regular rows of numerous small rounded tubercles (indicating pits on the 

 inner surface of shell). Anterior muscle-scar apparently shallow, oval, large, 

 situated in the centre of the anterior end ; posterior scar larger, situated near the 

 upper part of the posterior end. Umbo in cast preceded by a shallow concave 

 sulcus, running halfway down. Lunule biconcave, undefined. Shell-structure 

 very thin. 



Size. — Height 16 mm., length 35 mm., depth of one valve 5 mm. 



Localities. — In the Porter Collection is one specimen (cast and mould) from 

 Fremington; and in my Collection four specimens from Frankmarch. 



Remarks. — The Fremington specimen is larger and longer than the others, 

 and does not show the pittings, sulcus, and post-nmbonal ridge seen in the latter. 

 It seems, however, probable that they all belong to one species, and that the 

 differences are due to their state of preservation. The character of the hinge 

 seems peculiar and difficult to interpret in the different specimens. 



Affinities. — It closely resembles the two species of Sjwthella described by 

 Hall, 1 but differs in its more regular ornament, less cylindrical form, and narrower 

 and longer anterior end. It seems to bear the resemblance to Lithophagi indicated 

 by Hall. 



Pullastra modiolaris, F. A. Bomer, 2 is flatter, with a shorter anterior end, and 

 smaller, less defined umbo. 



3. Genus — Digoniomta (Provisional genus). 



Shell very inequilateral, angular, transversely rhomboidal, acute behind. 

 Hinge long, straight, thickened. Back depressed centrally, with rounded ridges 

 running from the umbo to the anterior and posterior ends of the base, posteriorly 

 oblique and flattened. Ligament external, situated in a long groove (?). Umbo 

 acute, proximate. 



This provisional genus is suggested with the greatest hesitation on account of 

 the extreme poverty of the material ; but it seems to be the only course open, as 



1 1885, Hall, ' Pal. N. Y.,' vol. v, pt. I, No. 2, pp. 407, 408, pi. lxvi, figs. 36—42. 



2 1850, F. A. Eomer, 'Beitr. Harzgeb.,' pt. 1, p. 60, pi. ix, figs. 21 a, b. 



