34 AUSTRALASIAN ANTARCTIC EXPEDITION. 
phenomenon has been described in M. craniwm by Fischer and Oehlert (1891, p. 72), 
while a straight front is also exhibited by M. craniella Dall (1895). On the other hand 
M. diamantina Dall has a well-marked ventral uniplication. 
The beak is short, and sub-erect to erect, as is usual in species of this genus. 
Beak ridges are not well-defined, but appear to point dorsally of the foramen, which is 
thus permesothyrid in position. The foramen, however, merges dorsally into an open 
delthyrium, with only very feebly developed lateral deltidial plates. | Within the beak 
there is a deposit of callus, resembling a pedicle collar, but closely applied to the bottom 
of the valve, passing up in front laterally into the dental plates, which support small 
teeth. A similar deposit of callus is described by Dall in M. americana, while in M. 
cranium it is figured, although not described, by Fischer and Oehlert (1891, plate V, 
fig. 10 f.). The deltidial plates are very feebly developed and less prominent than in 
M. americana. 
In the dorsal valve, the loop is long, extending forwards three quarters the length 
of the valve, when it is sharply recurved and gives off a few spinules on each side. The 
transverse band is narrow, its greatest width of the loop occupying only about a third 
of the breadth of the valve. The average breadth of the ribbon does not exceed one 
millimetre, except in the neighbourhood of the crural processes, which point’ ventrally 
inwards and are approximate. The crura are very short and spring from crural bases 
which are fused laterally on their outer margins with the short, widely diverging socket 
ridges. There is no median septum in the adult shell, while the hinge plate is represented 
by two lamellae which descend steeply from the inner sides of the crural bases to the 
floor of the valve a little more than one mm. on each side’ of the middle line, and are 
excavate anteriorly on their outer sides. These lamellae appear to be continuous 
across the floor of the valve along the middle line of the valve, closely applied to the 
bottom, extending back right to the neighbourhood of the umbo, and forward beyond 
their lateral free anterior edges for a slightly greater distance. In front of them there 
are three raised thread-like lines extending forward a short distance. 
The surface of the shell is smooth, with only occasional traces of a radial ornament 
such as is described in the next species. The lines of growth are mostly very. fine, but 
a few well-marked growth pauses may be seen on all the specimens. The shell substance 
is delicate. There are 80-112 pores per sq. mm., the pore density being somewhat 
variable on the same specimen, but a little lower than in M. vanhoffeni (120-132) though 
in view of the variability in this respect displayed by species of Macandrevia, too much 
importance must not be attached to such differences. 
The above described peculiarities in the beak characters, dental plates and 
cardinalia seems to be common to all species of Macandrevia,* and are alone sufticient to 
establish the generic position of any shell. The direct evidence of the Macandrevian 
type of moon Serenata is, however, ome Neo in the ea SUD a specimen, of 
* Cf. Thomson, 1916, No. 2, pp. >. 502-503, and the fe of M. vanhoffeni by Kichler (gu, Taf, XLH, fig. 9,d; 
Taf, XLII, fig. 14, 
