CLAVELLAT^E. 25 



separated ; the posteal muscular scar is unusually large. The height is equal to four fifths 

 of the length ; the diameter through the united valves is equal to half the length. 



Stratigra/phical position and locality. The Neocomian formation of Downham, 

 Norfolk ; the rock is a coarse, brownish, or sometimes greyish-brown, incoherent sand- 

 stone, locally called Carstone ; various specimens, for the most part ill-preserved, 

 and also external casts, have been liberally forwarded to me from the Museum of the 

 Lynn Philosophical Society, through the kindness of Dr. Lowe of that place. Our 

 figures are taken from moulds of gutta-percha pressed into the external casts, and also an 

 indifferently preserved internal cast. 



Trigoxia Juddiana, Lycett, sp. nov. Plate II, figs. 6, a, b, c; Plate IV, figs. 5, 7. 



Shell gibbose, ovately oblong, short and truncated anteally, posteally flattened and 

 angulated ; umbones antero-mesial, elevated, much incurved but scarcely recurved ; lower 

 border curved elliptically ; superior border of moderate length, slightly concave, termi- 

 nating posteally in a considerable angle with the wide posterior border of the area. 

 Escutcheon large and slightly depressed ; its superior border is not raised. Area wide, 

 mesial furrow conspicuous ; the superior half of the area is depressed concave ; marginal 

 carina small, but well marked, with a row of regular, small, distinct tubercles ; transverse 

 plications upon the area very irregular, often wrinkled; they frequently unite to form 

 varices at the median and inner carinae ; near to the apex they become regular, plain, 

 narrow, transverse costellse. The other portion of the valve has about twelve or thirteen 

 rows of clavellated costoe, which curve obliquely downwards and forwards from the 

 marginal carina, and form short, abruptly attenuated varices upon the curvature of the 

 anteal smooth, flattened space ; the rows terminate posteally at a smooth, slightly depressed 

 space, which widens downwards and separates the rows from the marginal carina. The 

 tubercles, from six to eight in each row, are prominent, pointed, and somewhat ovate ; 

 they are nearly of equal size, and the rows are symmetrical, excepting the two last 

 formed, which are rendered squamous by the large plications of growth near to the lower 

 border. 



The diameter through the united valves is equal to half the length of the marginal 

 carina. 



This is one of the most convex, and also short or sub-quadrate, forms of the 

 Clavellatce ; it will readily be distinguished by the general shortness of the figure, and the 

 truncated outline, both anteally and posteally, the smooth post-costal space, the general 

 gibbosity, and the short oblique rows of prominent pointed tubercles. It is allied to, but 

 I believe distinct from, a clavellated species found in the Kimmeridge Clay of Boulogne 



4 



