158 J. WILFRID JACKSON ON 



DALL 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 I have been enabled to study these points and make comparisons with the 

 Antarctic form. 



BALL'S type shows 104 to 112 punctse 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, Mull., = 188 to 272 (adults 192 to 216) ; M. 

 vanho/eni, Bloch., = 1 20-1 32. 



The 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 OEHLERT (1892), more especially fig. 28 of plate x. They are quite 

 distinct from those of Magellania venosa depicted by these authors (loc. cit., pi. xii. 

 figs. 5 and 1 5). 



(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 of the brachial 

 support, which is figured for the first time in this report (PI. II. fig. 16). 



In some samples, however, of deposit (No. 38) brought up later from the same 

 depth and station, I 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., 

 and shows the loop in its platidiform stage (PI. II. 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 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 380.) 



