NUCULA. 25 



Description. — Shell subtriangular or more or less oval, compressed ; posterior 

 part very short and somewhat angular ; anterior part usually tapering rapidly, 

 with the extremity rounded. Ventral margin curved evenly. Antero-dorsal 

 margin curved ; postero-dorsal nearly straight or slightly concave. Umbones not 

 prominent, carved. Lunule shallow, cordate, often not sharply denned. Anterior 

 area limited by a faint ridge. Surface smooth, but with lines of growth. Margin 

 smooth. Adductor impressions shallow. 



Measurements : 





(i) 



(2) 



(3) 



(4) 



(5) 



Length 



22 



17 



. 15 



. 14-5 



12 mm. 



Height . 



. 15-5 . 



12 



. 11 



. 10 



■ 10 „ 



Thickness 



. — 



— 



. 8 



. 6 



J5 



Affinities. — N. Timotheana, Pictet and Roux, 1 is shorter, and apparently more 

 compressed than is N. albensis; N. ovata, Mantell, is less compressed and more 

 rounded in outline; N. impressa, Sowerby, is more oval, and has a more depressed 

 and better defined lunule. 



Remarks. — This species varies considerably in the proportions of length and 

 height — the shorter examples being more triangular, the longer more oval in out- 

 line. A form found in the Blackdown Greensand (Plate IV, fig. 17), of which 

 I have seen only a few specimens, approaches very closely the shorter types of 

 N. albensis, but I have not sufficient material to allow me to speak of its affinities 

 with certainty. 



Types. — I have not seen the type ; it appears to be missing from the d'Orbigny 

 Collection in the Museum of Natural History, Paris. Some of the specimens 

 figured by Gardner (figs. 6 — 8) are in the British Museum. 



Distribution. — Lower Gault (zone vi) of Folkestone ; Gault of Black Ven ; 

 Cambridge Greensand (derived). ? Grey Chalk {fide Gardner). 



Nugula gaultina, Gardner, 1884. Plate IV, figs. 18 a—c, 19, 20, 21 a—c. 



1884. jVttcula gaultina, J. S. Gardner. Quart. Journ. Geol. Soc, vol. xl, 



p. 134, pi. iii, fig?. 9—11, 26, 27. 



Description. — Shell small, triangular, rather convex ; pointed anteriorly and 

 posteriorly. Ventral and antero-dorsal margins gently curved ; postero-dorsal 

 margin slightly concave. Umbones pointed, curved, placed very posteriorly. 

 Lunule cordate, sharply defined. Anterior area bounded by a ridge passing from 

 the umbo to the anterior extremity. Surface nearly smooth, but with lines of 



1 Moll. Foss. Gres verts de Geneve (1852), p. 470, pi. xxxix, fig. 7. 



4 



