CUCULL^A. 53 



Description. — Shell rather stout, rounded, nearly equilateral. Anterior 

 margin rounded; posterior a little truncated ; ventral curved. Umbones promi- 

 nent, incurved. Hinge-area relatively broad. Posterior part of shell sharply bent. 

 Ornamentation consists of strong, equal, regular ribs, separated by broader 

 grooves ; the grooves are crossed at regular intervals by fine concentric ribs. 

 Central teeth transverse; lateral parallel to the hinge-line. Margins of valves 

 coarsely toothed. 



Measurements : 



Length 20 mm. 



Height 18 „ 



Remarks. — This form, which at present is only known by a few rather 

 imperfect specimens, appears to be clearly distinguished by the character of its 

 ornamentation. 



Cucullsea erraus, Keeping, nom. nud., I think undoubtedly refers to this 

 species; the name was used in the earlier part of Keeping's work (vide supra), 

 and the remarks there made concerning it agree perfectly with those given later 

 in connection with C. vagans. This view is further supported by the fact that 

 on the back of the tablet which bears the type-specimens of C. vagans there is 

 written in Keeping's handwriting " C. errans, W. K." 



Types. — In the Woodwardian Museum, Cambridge. 



Distribution. — Lower Greensand (black grit nodules) of Upware. ? Spilsby 

 Sandstone of Spilsby. 



Sub-genus — Dicbanodonta, s.-g. nov. 



Shell stout, subquadrate or rounded. Hinge-area broad. Hinge-plate large, 

 curved ; central teeth transverse ; lateral teeth long, curved ventrally, nearly 

 parallel, often bifurcating. No posterior adductor plate. 



Type. — Cucullsea donningtonensis, Keeping (PL X, figs. 11 — 14; PI. XI, fig. 1). 

 Claxby Ironstone. 



The form described by Keyserling, 1 and by F. Schmidt, 2 as Pectunculus petschorse 

 probably belongs to this sub-genus. The latter author was inclined to regard it 

 as the type of a new genus. 



1 'Reise in das Petschora-land ' (1846), p. 306, pi. xvii, figs. 5, 6. 



2 "Resultate der Mammuthexpedit.," 'Mem. Acad. Imp. Sci. St. Petersbourg,' ser. 7, vol. xviii, 

 No. 1 (1872), p. 151, pi. i, fig. 14; pi. iiirt, fig. 17. 



