3 26 DISCOVERY REPORTS 



light of what has been learnt from the Discovery specimens, shows that it is distinguished 



from those of A. nuda by the features just enumerated. 



These three species can thus be distinguished by their basal heterozooecia as 

 distinctly as by other characters, and they form an interesting graded series in which 

 A. nuda has a true avicularium, A. benemunita an avicularium showing some of the 

 characteristics of a vibraculum, and A. rudis a true vibraculum. 



A median basal heterozooecium is commonly present at the bifurcation in these 

 specimens (Fig. 2 C) and a few are to be seen in the type specimen. Like the lateral 



mdh 



Fig. 3. A. Amastigia benemunita (Busk). St. WS 82, Falkland Islands. Basal view of a narrow branch with 

 large basal avicularium. The underlying zooecia are stippled, and the outlines of their basal walls 

 omitted for clarity. Three mandibles are shown, one being open and very short. The fourth hetero- 

 zooecium has an opesia, but no mandible. B. A. benemunita (Busk). Challenger St. 315. Young colony 

 with ancestrula. 34 . 1 1 . 1 2 . 47. 



mdb. mandible, ops. opesia of heterozooecium, r. rootlets, sc. scutum. 



basal heterozooecia, which are larger, it has a chamber of variable shape. The mandible 

 is articulated some distance from the distal end of the chamber and is directed straight 

 backwards (i.e. proximally along the branch). The distal prolongation of the chamber 

 is often visible in frontal view as a small triangular cell in the angle of the bifurcation. 

 Marginal avicularia, when present, are most commonly on the zooecia bordering the axil. 



