c 
K. L. Prendergast. 
Brachial Impressions. — The brachial impressions are similar throughout 
the group. In Strophalosia^ as already noted, they arise at the lateral 
extremities of the posterior adductors and continue to the lateral margin at 
an angle to the hinge-line varying from 4r)°-0°. They then run parallel or 
neaz’ly so to the lateral and anterior margins to a point about one-third of 
the width of the shell from the lateral margins when they curve through a 
right angle towards the hinge-line. This line is followed until the impressions 
are opposite the end of the median septum where they have another right 
angle bend to meet at the anterior end of the median septum. In Productus 
s.L and Aulosteges the brachial impressions arise at the anterior end of the 
dendritic impressions and continue to the lateral border in a course parallel 
to the hinge-line. When almost at the lateral margin they curve in a wide 
arc and return to the base of the median se])tuni, again running parallel to 
the hinge-line. In many specimens the returning arm is but faintly seen or 
may be totally obscured. The interpretations of the brachial impressions 
have been numerous. They are now described as the attachments of the 
brachial arms which ])rojected ventrally from them. Muir-Wood (1928, p. 20) 
has cited some evidence in favour of this view from the minute structure of 
the impressions. The upward medial bend, where the impressions recede 
from the anterior margin would correspond, then, to the pallial sinus of the 
Lamellibranchs, being the region of water intake. The water would bo ejected 
from the apices of the cones and this current would thus have no effect 
on the basal attachments. The development of a fold in the brachial valve 
with the corresponding sinus in the pedicle valve would serve further to 
localise the ingoing and outgoing currents. 
Cardinal Process. — In the description of the species the term ^Trifid’’ 
has been used to describe the cardinal process. To avoid misapprehension 
that term is here defined as referring to a cardinal process which is bilobed 
in ventral aspect and trilobed in dorsal aspect or which is a modification of 
this type. The dorsal and ventral aspects are the views of the cardinal 
])rocess as seen from the brachial and pedicle valves respectively. 
To expand this definition the cardinal process of Aulosteges wangenheimi 
(Vern.) may be considered (refer to Fig. 5, p. 34). In this species the 
cardinal process is perpendicular to the plane of the brachial valve and is 
supported by two lateral ridges arising from the cardinal ridge. These 
ridges, which are separated medially by a deep sulcus, give rise to two 
convex masses, the lobes of the ventral aspect, separated by a depression. 
The dorsal aspect shows the structure of the process clearly. It has the 
form of a double-S. The median convexity of this double-S is the central 
lobe, and the end arms the lateral lobes of the dorsal aspect while the two 
concavities are the lobes of the ventral aspect. The whole process has the 
form of an open and indented cone with its apex at the hinge-line. The 
growth-lines of the structure may be followed round the dorsal face and 
are ])arallel to the base of the cone. 
AH the cardinal processes examined were modifications of this type. 
Thus in Aulosteges haracoodensis Eth. fil. the process is not in such an 
advanced stage of development. The dorsal aspect shows the process elon- 
gated with the lobes becoming distinct some distance from the hinge-line. 
The ridges of the ventral aspect have remained as ridges and are not enlarged 
into the convexities seen in Aulosteges wangenheimi (Vern.). They are 
indented posteriorly having a quadrilobed termination. 
