Upper Silurian. ] PALMONTOLOGY OF VICTORIA. [ Mollusca. 
PratE XLVI., Fra. 1. 
LEPTANA (LEPTAGONIA) RHOMBOIDALIS (Wick. sp.). 
[Genus LEPTONA (Dal.). (Sub-kingd. Mollusca. Class Palliobranchiata. Ord, Brachio- 
poda. Fam. Orthiside.) 
Gen. Char—Semicircular, depressed, entering valve concave, the other evenly convex from 
the straight hinge-line, which is as long as the shell is wide; cardinal area distinct, slightly 
rhomboidal, about equally formed of both valves; the convex or receiving valve has a triangular 
opening, from the sides of the base of which diverge two strong cardinal tooth ridges, the 
aperture nearly filled by the projecting angle of a rhomboidal, two or four lobed, boss at the 
beak of the entering valve, leaving only a minute perforation, separated by a small pseudo- 
deltidium from the apex; punctured structure minute, often indistinct or obsolete; internal 
surface rough with little points; entering valve with a small bifid tooth at the beak, in front of 
which are two small diverging socket ridges, between which is a moderate longitudinal septum. 
Some species show a minute tubular foramen at the apex of the beak, others are closed. 
This genus may be said to embrace three sections or subgenera not worthy I think of generic 
rank, on careful study of a very extensive suite of specimens of most of the known species. 
These are :—Leptena Dalman, restricted to the type of his last species, the Z. transversalis, in 
which the valves are almost equally curved in the same direction, the receiving or foraminated 
one convex; the other concave outwardly, and having the muscular impressions and ovarian 
spaces larger than in the others. 2nd. Strophomena, in which the valves are flat or very slightly 
convex when young, the margin in a few species becoming by age deflected usually towards the 
receiving valve, e. g. L. euglypha (Dal.) and Strophomena rugosa (Raf.). 3rd. Leptagonia (McCoy), 
with both valves abruptly bent at right angles towards the entering valve, and the rostral 
portion concentrically wrinkled. In former works I included Chonetes (Fischer) amongst the 
ce aan of Leptena, but, in deference to the opinion of de Koninck and Davidson, I now 
omit it. 
DescripTion.—Square prismatic ; rostral portion quadrate ; sides and front 
margin nearly straight, slightly undulated; slightly convex near the beaks; 
irregularly concave before reaching the angle, which is seldom as high as the 
surface near the beak ; concentric wrinkles, about twelve to fifteen, strong, rounded, 
slightly undulated, and some of them occasionally interrupted in parts, following 
the direction of the margin, and turning abruptly outwards near the cardinal line ; 
deflected front prismatic; sides sub-parallel, or slightly converging towards the front 
margin, which is flattened, or with few irregular longitudinal folds, dilated at the 
cardinal angles into flattened ears; longitudinal striz coarse, obtuse, equal, about 
nine to twenty (usually twelve) in the space of 2 lines at the edge of the rostral 
portion ; interior of receiving valve with two short cardinal teeth, diverging at 115°, 
from the ends of which a slender prominent ridge curves forwards and inwards on 
each side, forming the boundary of the rather large, ovate, adductor impressions, 
which reach about half the length of the rostral portion, the two being separated by 
a mesial ridge extending rather less than their length; interior of entering valve 
with the rostral portion very concave, and sharply defined from the deflected front 
by a prominent narrow margin ; rostral tooth very large, and deeply bifid ; the pits 
for the teeth of the opposite valve are flanked anteriorly by two thick ridges, which 
gradually incurve, forming the tubercular boundary to the rounded posterior pair of 
muscular impressions ; anterior to which are the much smaller anterior pair of ovate 
impressions, with tumid boundaries; the mesial separating ridge extending two- 
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