836 



MICROSCOPIC FORMS OF ANIMAL LIFE 



two different kinds of structure are usually seen in it, one being 

 composed of chamberlets of very definite form, quadrangular in some 

 species, circular in others, arranged with a general but not constant 

 regularity in concentric circles (figs. 636, 637, b, b) ; the other, less 



FIG. 637. Portions of the section of Orbitoides Fortisii, shown in fig. 636, 

 more highly magnified : a, superficial layer ; b, median layer. 



transparent, being formed of minuter chamberlets which have no 

 such constancy of form, but which might almost be taken for the 

 pieces of a dissected map (a, a). In the upper and lower walls of 

 these last, minute punctations may be observed, which seem to be 



FIG. 638. Vertical section of Orbitoides Fortisii, showing the large 

 central chamber at a, and the median layer surrounding it, 

 covered above and below by the superficial layers. 



the orifices of connecting tubes whereby they are perforated. The 

 relations of these two kinds of structure to each other are made 

 evident by the examination of a vertical section (fig. 638), which 

 shows that the portion b, figs. 636, 637, forms the median plane, 

 its concentric circles of chamberlets being arranged round a large 

 central chamber, as in Cycloclypeus ; whilst the chamberlets of the 



portion a are irregularly superposed one 

 upon the other, so as to form several 

 layers which are most numerous towards 

 the centre of the disc, and thin away 

 gradually towards its margin. The dis- 

 position and connections of the cham- 

 berlets of the median layer in Orbitoides 

 seem to correspond very closely with 

 those which have been already described 

 as prevailing in Cycloclypeus, the most 

 satisfactory indications to this effect 

 being furnished by the silicious ' internal 

 casts ' to be met with in certain Green- 

 sands, which afford a model of the sar- 

 code-body of the animal. In such a 



fragment (fig. 639) we recognise the chamberlets of three successive 

 zones, a, a f , a", each of which seems normally to communicate by 

 one or two passages with the chamberlets of the zone internal and 

 external to its own ; whilst between the chamberlets of the same 



FIG. 639. Internal cast of por- 

 tion of median plane of Orbi- 

 toides Fortisii, showing, at 

 a a, a' a', a", a", six chambers 

 of each of three zones, with 

 their mutual communications ; 

 and at b b, b' b', b" b", portions 

 of three annular canals. 



