38 HENRY B. BRADY. 



sisting of a cylindrical arenaceous tube, with swollen centra 

 chamber. Tubular portion often irregular in outline, taper- 

 ing towards the ends ; shell-wall of the central chamber 

 thinner than that of the two arms. Length ^ inch. 

 (4'5millim.). 



Amongst the types of arenaceous Rhizopoda enumerated 

 by the lamented Scandinavian naturalist, Michael Sars, in 

 the short paper summarising the results of his deep-sea 

 researches/ is a fine large species, which he names Rhahdam- 

 mina ahyssorum. In the absence of any description from the 

 pen of the discoverer it may be characterised as having a 

 radiate test, consisting of three to five long arenaceous tubes, 

 diverging from the central point like the spokes of a wheel. 

 In specimens from some localities there is a small central 

 chamber, and in these cases the arms are broad at the point 

 of insertion and somewhat tapering, but more frequently the 

 arms are of nearly even diameter, and there is little or no 

 swelling at the point of union. Generally speaking, the 

 tubular portions radiate on one plane and the test is com- 

 planate, but sometimes this order is not observed, and they 

 diverge irregularly. Specimens of Widbdammina ahyssorum 

 from the southern hemisphere do not differ in any important 

 particular from those obtained by Sars from a depth of 450 

 fathoms off the Norwegian coast. Under favorable conditions 

 the species attains a considerable size, but owing to the 

 tenuity and brittleness of the rays it is seldom, probably, 

 that specimens are quite perfect \ examples, however, are 

 not uncommon that must, when complete, have measured an 

 inch from point to point. 



In three or four of the '' Challenger" dredgings there is 

 found a much smaller form referable to the same genus, but 

 with sufficiently distinctive characters of its own, and this I 

 propose to name Mhabdammina linearis. It may be regarded 

 as a two-rayed modification of the type, with a central in- 

 flated cavity. The two arms are seldom of the same dia- 

 meter, nor are they usually set on so as to form a right line. 

 In texture the test is more loosely built, and the sand-grains 

 less completely incorporated, than in the typical species. In 

 light-coloured specimens the extremities are sometimes stained 

 reddish brown. 



The mere fact of possessing but two arms instead of three, 



four, or five, would not by itself constitute a valid reason for 



distinguishing a variety by name, especially if it were found 



in company with the radiate form ; but, taken in conjunction 



1 ' Vidensk.-Selsk. Forhandinger' for 1868. 



