536 ME. T. H. WITHERS ON 



margin slightly concave and about the same length as the tergal 

 margin, the two margins if represented by lines from the apex to 

 the lateral angles would enclose an angle of about 55° ; basal 

 margin rovmded, and indistinctly marked off into three lines ; 

 a portion of the valve on either side, parallel with the scutal and 

 tergal margins, is somewhat raised, and the lines of growth on 

 these parts are upturned sharply towards the umbo. Surface of 

 va,lve between the raised portions ornamented with several longi- 

 tudinal ridges. 



Rostral latus. — This valve is imperfect, the upper portion being 

 broken off. Valve very narrow, acutely angular transversely, 

 about 2| times as long as wide, widening gradually from the 

 inner acute extremity to the rostral margin, which is abruptly 

 truncate ; umbo apical ; the inner extremity is marked by a 

 strong rounded keel ; outer (rostral) half of valve ornamented 

 with fine longitudinal ridges, two fine ridges close together almost 

 dividing the basal margin into two equal portions. At the point 

 where these two ridges reach the basal margin the valve is some- 

 what convex, no doubt indicating the extent to which the valve 

 was bounded by the inframedian latus. 



Rostrum sub -triangular, strongly convex transversely, bowed 

 inwards ; lateral margins bounded by strong ridges ; basal margin 

 convex ; a strong median rounded keel extends from the apex, 

 widens considerably towards the basal margin, and is bounded 

 on either side by indistinct longitudinal ridges. 



Peduncle scales varying in shape from semikinar to trapezoidal, 

 the basal margin of the former being straight, while that of the 

 latter is somewhat concave ; immediately below the apex the 

 trapezoidal scales are slightly constricted, the truncated top 

 appearing to overhang ; scales thickest at one-third from the base, 

 below which, on the inner surface, they are somewhat excavated, 

 this indicating the extent to whicli the scales were covered by the 

 corium, the upper two-thirds no doubt overlapping the contiguous 

 scales. Outer surface ornamented similarly to the valves of the 

 capitulum with a number of longitudinal ridges, These ridges 

 number about seven on the larger scales, and four or five on the 

 smaller scales. 



Measurements. — Owing to the fact that the valves in these two 

 specimens of S. arcuatitm are broken, it is impossible to give 

 their accurate measurements. Since, however, it is desirable 

 that we should have some idea of the relative sizes of the valves 

 in an individual, approximate measurements ax^e given where the 

 correct measurements cannot be obtained. 



Specimen I. 13577. 



Carina. Length circa 21 mm. 



Scutum (right valve). Length (from apex to i-ostral angle) 

 13 '5 mm. ; breadth 7*4 mm. 



Tergum. Length circa 17 mm, ; breadth 8*2 mm. 



