Mr. T. R. Jones on some species of Lejjerditia. 85 



ing into the cavity of the carapace. The line of junction of the 

 inverted border, or ventral plate, and the outer surface of the 

 valve is angular, and bears a slight sulcus and moulding, against 

 vfhich. the overlapping edge of the right valve abuts. The dorsal 

 or the ventral profile of the closed valves is elongate acute oval ; 

 the end view of the closed valves is more or less ovate. 



Rather above and in front of the centre of each valve, and on 

 its most convex portion, nearly all the species of the genus pre- 

 sent a slightly raised, circular or suboval swelling, having a dia- 

 meter of from j^th to ^th of the breadth of the valve^s surface. 

 This swelling is distinguishable by a local change of colour or 

 of surface-condition, and marks the place of a corresponding 

 rounded pit, excavated on the interior surface of the valve so 

 deeply as to render the tissue of the valve at the swelling some- 

 what diaphanous. The cast of this pit on an inner cast of the 

 valve is strongly marked, having a greater relative height than 

 the external swelling. The swelling has usually a reticulated 

 appearance resulting from vascular impressions on the test : and 

 from a slight sulcus at the margin of the pit a set of delicate 

 canals*, tortuous and inosculating, excavated on the interior 

 of the valve, radiate forwards, downwards, and backwards, gra- 

 dally becoming fainter towards the edges of the valves. 



Anterior to the central tubercle, or " lucid spot,^' and nearer 

 to the dorsal margin, is a smaller, but prominent, tubercle on 

 each valve, with a corresponding internal pit. This little tubercle 

 (the " anterior tubercle ^') is usually seated on or at the edge of 

 a slightly raised area of irregular outline ; and behind it a short 

 shallow vertical sulcus, commencing at the dorsal margin, is 

 usually apparent. 



1. Leperditia Balthica, Hisinger, sp. PL VI. figs. 1-5. 



Cytherina Balthica, Hisinger, Anteckningar i Phys. o. Geol. 

 part 5. pi. 8. fig. 2; Lethsea Suecica, p. 10, 118. pi. 1. fig. 2 a, b, 

 pi. 30. fig. 1. 



Dimensions of closed or of separate valves of five individuals : — 



* These are noticed also by Count Keyserling in a species from 

 Petschora-Land. 



