380 J. WILFRID JACKSON ON 
Dau however, gives no particulars of the shell-mosaic and the number of pores per 
square millimetre. Through his generosity in lending me the pedicle valve of his type 
specimen | have been enabled to study these points and make comparisons with the 
Antarctic form. 
Dat’s type shows 104 to 112 punctze per square millimetre. In the ‘“‘ Coats Land” 
adult examples these range from 92 to 110, with an average of 99. 
Other known species: M. cranium, Mill.,=188- to 272 (adults 192 to 216); M. 
vanhoffen, Bloch., = 120-132. 
‘he shell-mosaic in both forms is practically identical, and consists of the usual 
overlapping scale-like structure. 
Several of the specimens exhibit very clearly the vascular sinuses in the pallium. 
In the dorsal valve there are two; these curve round the adductor muscular 
impressions and then diverge widely from each other, ceasing some little distance from 
the lateral margins of the valve. In the specimen examined these sinuses do not appear 
to bear any ramifications. 
In the ventral valve there are four sinuses; the two median ones almost straight, 
slightly diverging near their anterior extremities, somewhat broad posteriorly and 
narrowing gradually towards the anterior, where they end abruptly without ramifica- 
tions. These terminate some little distance from the anterior margin of the valve. 
The two lateral sinuses are slightly arched and send off four or five ramifications on 
their exterior sides, two or three of which again subdivide near the extreme lateral edge 
of the valve. These two sinuses are connected with the two median ones at a point about 
a quarter the length of the valve, whence they diverge. 
Compared with the pallial sinuses of Terebratella dorsata, those of the ventral 
valve of M. diamantina present a striking resemblance to the illustrations given by 
FiscHER and OErHLERT (1892), more especially fig. 28 of plate x. They are quite 
distinct from those of Magellana venosa depicted by these authors (oc. cet., pl. xii. 
figs. 5 and 15). 
(Note.—The dorsal and ventral valves referred to above are not of the same 
individual. ) 
As previously mentioned, the examples brought up by the trawl at this station 
were in an adult condition; they all exhibit the final development vf the brachial 
support, which is figured for the first time in this report (PI. Il. fig. 16). 
In some samples, however, of deposit (No. 38) brought up later from the same 
depth and station, | was pleased to find two examples of this species which exhibit 
interesting stages in the development of this organ. 
The smallest specimen measures (dorsal valve): length, 4 mm.; breadth, 4 mm., 
aud shows the loop in its platidiform stage (Pl. Il. fig. 17). It here consists of two 
descending branches, which converge towards the centre of the valve, where they 
become attached to a laterally compressed tube-like septal pillar possessing a few spinous 
processes on its anterior edge. This stage agrees almost exactly with the figure of the 
