72 EDITH M. PRATT, B.SC. 



Porella tridentata, sp. nov. 



Zooecia hexagonal, separated by fairly deep linear furrows ; surface 

 convex and punctured by large irregular pores which, in some cases, 

 are definitely arranged round the margin. 



Secondary orifice horse-shoe shaped or inversely subtriangular, with 

 a lateral raised collar meeting in a sudden depression behind (Fig. 4, b), 

 often a sinus in front, containing a small avicularium with a rounded 

 mandible (Figs. 4 and 5, b). In many cases a spatulate avicularium 

 is present to the right of the secondary orifice (Figs. 4 and 5, a). 



Deep down in the orifice can be seen a large median denticle with 

 two lateral ones (Figs. 4 and 5, c and d) ; because of this feature it has 

 been thought fit to call the species " tridentata." 



The ovicell (Fig. 5, Ov.) is semicircular in outline, convex in front, 

 somewhat granular, with one or two groups of irregular punctures. 



Zoarium encrusting shell (Photinula violacea) and of a dirty pink 

 colour. 



Porella tridentata is somewhat like P. concinna (Hincks, Brit. Mar. 

 Polyzoa, p. 323). It differs from the latter in having two lateral 

 denticles in addition to the median one which is present in P. concinna. 

 It is also somewhat larger, and the pores are bigger and irregular in 

 shape. The spatulate avicularium appears to be constant in position, 

 while the general shape of the secondary orifice seems to be somewhat 

 different. 



P. tridentata differs from Jullien's P. malouinensis (see Cap Horn 

 Exp'ed., p. 57) in that the pores are larger and fewer in number. P. 

 malouinensis appears to have no spatulate avicularium or denticles, and 

 the secondary orifice is somewhat different. The ovicell agrees with 

 Julliens description of that in P. malouinensis, which, however, he does 

 not figure. 



A consideration of the geographical distribution of the Bryozoa in 

 this collection from the Falkland Islands, is of special interest at the 

 present time because of the controversy about the origin of the north 

 and south extra-tropical marine faunas. 



Murray 1 is of opinion that " if there were once a nearly universal 

 climate over the whole of the ocean, then it is possible that there was 



1 Murray " On the Deep and Shallow-water Marine Fauna of the Kergnelen 

 Region of the Great Southern Ocean." {Trans. B. Soc. Ed., Vol. 38 (1896), 

 p. 343 ; also Challenger Summary of Results). 



