PRESIDENTIAL ADDRESS. 587 



chamber reveals a row of foramina along the line where the chamber, in- 

 cluding its alar prolongations, rests against the whorl which it bestrides. It 

 results from what has been said that they present a V-shaped line. These foramina 

 are the main openings by which the cavity of the last chamber opens to the 

 exterior. Each chamber of which the shell is composed has been in its turn the 

 terminal chamber, and the openings which then led to the exterior subsequently 

 form communications leading from chamber to chamber. As we trace them back to 

 the earlier chambers they become fewer in number until only a single foramen is 

 found between the chambers. In specimens of the type we are considering a 

 comparatively large globular chamber is the starting point from which growth 

 proceeded, A short passage leads to the second chamber, whicb has a peculiar 

 shape, being applied to the sphere, produced at one end into a point, and abutting 

 at the other against the third chamber. From this onwards the typical shape is 

 gradually assumed, though in these earlier chambers the alar prolongations are 

 absent. A character of this genus is the presence of the line of pocket-like 

 processes along the posterior margin of the chambers. It was not clear, until 

 Williamson's paper was published, that these ended blindly and did not communi- 

 cate with the chamber behind. The outer walls of the chambers are traversed by 

 multitudes of pores of extreme minuteness, so that the chambers of the outer 

 whorl have this additional means of communication with the exterior. There is, 

 besides the structures described, a system of canals, lying in the thickness of the 

 walls, and communicating with the chambers, but this need not detain us here. 



It results from the structure of such a form as Polystomella that in the 

 earliest stage of its existence the whole organism consisted of a single spherical 

 chamber. 



It is to be observed that in shells such as Polystomella the shape and mode of 

 growth of the organism at all stages of its development are preserved in the 

 central parts of the shell. These early formed chambers may be, in some types of 

 growth, exposed to observation, or they may be, as in this genus, built in and 

 hidden by the overlapping of the subsequent additions. They may then, however, 

 be examined by making sections of the shell, or in the protoplasmic casts of the 

 interior when the shell is dissolved. 



The Foraminifera are found living attached to other objects on the sea bottom 

 from shore pools down to great depths, and fi'om arctic to tropical waters. A 

 small group of them lead a pelagic life suspended in the upper layers of the great 

 oceans, from the surface down, as Dr. Fowler's collections from the Bay of Biscay 

 show, to at least 600 fathoms, and their empty shells falling to the bottom consti- 

 tute the large proportion of the grey ' Globigerina ooze' which in many regions 

 forms the floor of the ocean. 



An attractive feature of their study is the abundance with which they are 

 represented in geological deposits, right back to the Palaeozoic period, so that in 

 dealing with them we have that third dimension, the history of the group in the 

 past, wide open to us in which to project our ideas of the course of their evolution. 



It was from the study of fossil Foraminifera of the early Tertiary period that 

 the recent advances in our knowledge of their life-history received its impulse. 



The later Eocene rocks in many parts of the world abound in discoidal, slightly 

 biconvex Foraminiferous shells, which, from their likeness to coins, have been 

 called Nummulites. The Nummulitic limestones extend across the Old World 

 from the Pyrenees to China, and often attain a thickness of thousands of feet. 

 Visitors to Egypt are familiar with them in the blocks of which the pyramids of 

 Qizeh are built, and the glittering coin-like discs polished by wind and sand and 

 strewn in the desert have attracted notice from remote antiquity. 



The structure of a Nummulite is very similar to that of Polystomella, but the 

 most spacious part of each chamber lies in the median plane of the shell, while 

 the alar prolongations are very thin and interrupted by supporting pillars of 

 solid shell substance. Hence the median plane is a plane of weakness, and the 

 shell readily splits into plano-convex halves, the broken surfaces exposing a 

 section in^the median plane of all the chambers of which it is built. 



