Monn One: | Ostracodous and Phyllopodous Tribes. 189 
borders by a simple linear hinge ; the two extremities of the hinge- 
border form angles with anterior and posterior borders in each 
valve. The richt valve is larger than the left, being broader, and 
overlapping completely the ventral border of the opposite valve, and 
to some extent its anterior and posterior borders. The overlapping 
ventral border of the right valve forms a thick blunt keel to the 
closed carapace. Each valve is somewhat depressed towards the dor- 
sal border ; this border in the left valve is thicker than that of the 
right, and sometimes slightly overrides it. In one species at least* 
the dorsal edge of the left valve swells out considerably in the 
middle. The ventral margin of the left valve is turned suddenly 
inwards, forming a thin plate projecting into the cavity of the cara- 
pace. The line of junction of the inverted border, or ventral plate, 
and the outer surface of the valve is angular, and bears a shght 
sulcus and moulding, against which the overlapping edge of the 
right valve abuts. The dorsal or the ventral profile of the closed 
valves is usually 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 present a 
slightly raised, circular or suboval swelling, having a diameter of 
from 3th to ith 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 somewhat 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. This has “usually a 
reticulated appearance, resulting from vascular i 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, gradually 
becoming fainter towards the edges of the valves. This central tu- 
bercle, “Sueid- -spot,” or “ muscle-spot,” marks the spot to which the 
great internal, closing or adductor muscle was attached. Anterior to 
it and nearer to the dorsal margin, a smaller but prominent tubercle 
is often visible on each valve, with a corresponding internal pit. 
This little anterior tubercle, or “ eye-spot,’ corresponded to the 
place of the eye in the animal, and is usually seated on or at the edge 
of a slightly raised area, irregular in outline ; and behind it a short 
shallow vertical sulcus, commencing at the dorsal margin, is usually 
apparent. 
In the ‘Annals Nat. Hist.” 1856, ser. 2, vol. xvi., pages 96, 
&c., when first treating of Leperditia, T offered some observations on 
* TL, gibbera, Jones, ‘Ann. Nat. Hist.,’ ser. 2, vol. ae , p. 90, pl. 7, figs. 8-10 ; 
and ibid., ser. 3, vol. i., p. 250, pl. 10, figs. ls 10, and 1 
