1907.] on the Foraminifera. 497 



[Two lantern slides were then displayed sliowino- the whole test of 

 a specimen of Orbitolites tenuissma,iind an eiilarii-ed view of its central 

 chambers.] 



Orbltolifes teiumsima is a frajG^ile shell occnrrino- at great depths. 

 The i)ei'ii)heral parts a^^'ree with those of O.nmrf/malis, bnt the central 

 region is built ou an entirely diiferent plan— resembling that of the 

 Miliolid genus Opldhahnidiiim. 



On the completion of the latter an astonishing change in the mode 

 of growth occurs, the shell being henceforth built on an entirely new 

 {)lan. One can hardly suppose that such a sudden abrupt mutation 

 befell the race, as is here recorded, and it would appear that some 

 other, though entirely unknown factor or factors, have contributed to 

 the result. 



The second case is that of Pohjtrema mlniaceum^ a pink encrusting 

 arborescent form, often seen attached to corals, polyzoon stocks, and 

 other objects from warm shallow waters. 



[A diagram of Pohjtrema miniaceum., with enlarged drawings of 

 the initial chambers, and a lantern slide exiulHtiug a section of Foly- 

 trema^ passing through the group of rotaloid chambers were exhibited.] 



In its mode of growth and the absence of any distinct chambers 

 throughout the greater part of its mass Polytrema differs widely from 

 most other Foraminifera, and it Avas referred to various groups of 

 animals by the earlier naturalists. By Pallas and Gmelin, it was 

 placed near the genus Millepora, which is now included, with corals 

 and hydroids, in the Coelenterata. Its true position was recognised 

 by the discovery that its growth begins by the formation of a small 

 spiral group of chambers with thick walls and coarse perforations such 

 as are characteristic of the foraminiferan family RoUtUdm. In tbis 

 case at any rate, there can be no question that the plan on Avliich the 

 first formed chambers are laid down is that of the stock from which 

 Polytrema sprang. 



In conclusion, the lecturer alluded to the relation in size in the 

 genus NummaMes between the megalosphere, and the volume of the 

 protoplasm of the microspheric parent, by the division of which it 

 arose, in the manner described. He also pointed out the analogy 

 presented by the life-history of those species of nummulites (e.g. N. 

 romplanatas), in which the microspheric form far exceeds the megalo- 

 spheric in size, to the life-history of a fern, in which the prothallus 

 is likewise dwarfed by the fern plant — the individual of the sexually 

 produced sporophvtic generation. 



[J. J. L.] 



2 K 



