HICKSON— POLYTREMA 457 



Hie structure of the shell. In comparing the three genera, attention may be called 

 to the relative hardness and density of the skeleton. In Sporadotrema and particularly 

 in the " Providentise " facies of the genus and in S. mesentericum the skeleton as a whole 

 is very hard and rigid. This hardness of the skeleton may be expressed by the use of 

 Carter's words " Consistence stony." In Polytrema on the other hand the consistence of 

 the skeleton is very brittle. It may be easily crushed into fragments between the finger 

 and thumb. In Homotrema the consistence of the skeleton is intermediate between that 

 of Sporadotrema and Polytrema. 



This difference in consistence is due to the difference in structure of the three genera. 

 The structure of the branches of Polytrema is very difficult to understand when taken by 

 itself. The structure of the branches of Homotrema and Sporadotrema seem to me to 

 throw light upon it and render its understanding more easy. If the growing end of a 

 branch of a Sporadotrema is examined it will be found to consist of a circle of more or less 

 biconvex or almost spherical chambers arranged on edge at the tip of the branch. The 

 convex surface facing outwards of each of these chambers is perforated by foramina, the 

 convex surface facing inwards is not perforated by foramina (Text-fig. A). The free edge of 

 each chamber is produced into three or four tubular processes sometimes arranged like the 

 points of a cock's comb (Plate 31, figs. 15 and 16). In well-preserved specimens one or more 

 of these tubes has a trumpet-shaped mouth the lips of which are the beginnings of a new 

 chamber. Tubular processes similar to those at the edge are sometimes situated on the 

 inner convex surface. Surrounded by these terminal chambers there is an interlocular 

 space (Text-fig. A i). 



The structure of the branches of Polytrema is far more difficult to understand, and 

 the descriptions given of it by Schultze, Mobius and Merkel are not consistent. After 

 careful observation of several specimens from different parts of the world I am quite 

 convinced that there is a wide range of structure of these parts and that a new series of 

 investigations based on the study of a large number of specimens is very desirable. There 

 can be no doubt that zoologists who have given their attention to the Foraminifera have 

 been inclined to "lump" all the Polytremidae into one species. I believe that when the 

 detailed structure is more carefully examined there will be a swing of the pendulum and 

 the genus Polytrema will be split into a large number of species. 



However, I will in this statement refer only to one or two points upon which there is 

 inconsistency of statement in the descriptions of previous writers and then describe what 

 I believe to be the structure of the growing point of specimens of Polytrema miniaceum 

 from the Mediterranean. 



Mobius describes in the branches of specimens of Polytrema he obtained in Mauritius 

 a central canal (eine centrale Rammer) around which the chambers are arranged spirally 

 or in circles. Merkel denies the existence of a true central canal but describes excentric 

 canals and spaces opening to the exterior with imperforate walls (Scheidewiinde) formed 

 by the fusion of the pillar walls. Lister writes of " axial spaces " which open widely at 

 the ends of the branches. 



The spaces with imperforate walls, sometimes opening to the exterior at the end of 

 the branches, can be clearly distinguished in some specimens (Text-fig. B i); but in a very 



