FORAMINIFERA. 117 



shell is spiral and discoidal, with spur-like marginal append- 

 ages, and with a well-developed " supplemental skeleton " 

 and " canal-system." The genus has been shown by Brady 

 to commence in the Carboniferous. In Planorbulina the shell 

 is composed of numerous segments, at first spirally and then 

 cyclically disposed. It dates from the Tertiary period, but 

 the forms which are included under the sub^generic name of 

 Truncatulina (fig. 18, p) commence in the Carboniferous. 

 Tinoporus, dating from the Chalk, is in some respects inter- 

 mediate between Calcarina and Planorlulina, its general form 

 being like the former, while the irregular and partly cyclical 

 arrangement of its chambers recalls the latter. There is 

 also sometimes a " supplemental skeleton " and " canal-sys- 

 tem." We may just mention, also, the genus Polytrema, 

 though not yet known in the fossil state, since it has some 

 curious resemblances to some forms of corals and Polyzoa. 

 It forms crusts, or, more commonly, branched outgrowths, 

 parasitically attached to foreign bodies ; and it consists of 

 numerous intercommunicating irregular chambers, the walls 

 of which are penetrated by an extensive system of capillary 

 canals. Polytrema seems to be the representative in the 

 Eotaline series of the singular genus Stacheia among the 

 Inperforata. Lastly, the genus Involuting from the Lias, is 

 usually placed among the Eotalines, though it presents some 

 peculiarities which would remove it from this series, or would 

 even place it altogether outside the section of the Perforate 

 Foraminifera. 



Finally, we have the family of the Nummulinida, comprising 

 the most complex and the most highly organised of all the 

 Foraminifera. In the forms included under this head, the 

 shell is compound, the successive chambers are enclosed each 

 in its proper wall (as diagrammatically shown in fig. 17, A), 

 there is almost always a well-developed " intermediate " or 

 " supplemental " skeleton, which renders the shell strong and 

 compact, and which is perforated by a " canal-system," origi- 

 nating in the spaces between the two lamellae of which each 

 septum is composed ; while the shell-substance is pierced by 

 close-set and extremely fine tubules, the septa alone wanting 

 these, so that contiguous chambers usually communicate by 



