part 'S'] A^D STEUCTUEE OF THE FOEAMIXIEERAL SHELL. 209 



The fundamental character b}^ which the porcellanous is distin- 

 guislied from the vitreous shell is its fhiely granular structure : and 

 all porcellanous shells are imperforate, though it h}^ no means 

 follows that all imperforate shells are porcellanous. 



The subdivision of the Foraminifera into the two groups Per- 

 forata and Imperforata has lately fallen into disrepute, owing to the 

 discovery among the existing Imperforata of examples which are 

 not devoid of perforations at an early period of their existence. 

 Thus, according to Rhumbler,i the embryonal chamber of Peneroplis 

 pertusiis is distinctly perforate, the pores extending all over it; and 

 not only so, but (according to Gr. Schacko ~) pore-canals occur also 

 on the septal sutures of this foraminifer, the perforations being close 

 and tine, and comparable with those of Nodosaria. It is further 

 affirmed by Dr. J. J. Lister-^ that the central cl. amber and spiral 

 passage of the megalospheric form of Orhiculina and Orhitolites 

 marginafa are perforate. 



On these rather slender grounds Khumbler maintains that the 

 terms ' Perforata ' and ' Imperforata ' are no longer applicable, and 

 Prof. O. Abel ^ proposes as substitutes ' Porcellanea ' and ' Vitreo- 

 calcarea ' : of these terms the latter is cei'tainlj'' open to objection, 

 for, since both groups are essentially calcareous, it is not only 

 redundant but to some extent misleading. 



On the general question of classification there is room for 

 difference of opinion, and I may commence such observations as 

 I have to make by calling attention to the three isomorphous 

 genera, ConiKspira, Spirillina, and Amnodiscus, which (with the 

 doubtful exception of Spirillina) are devoid of perforations and 

 yet respectively porcellanous, vitreous (though in an unusual 

 manner), and agglutinating. Thus the imperforate character 

 Avould seem to be more constant than the structure or composition 

 of the shell. 



Unfortunately, the palaeontological record affords less information 

 than we could wish, but it may be remarked that Rhumbler's 

 attempt to derive the calcareous from the arenaceous Foraminifera 

 is directly opposed, so far as it is based on palaeontological evidence, 

 by Mr. F. Chapman's account of the oldest known Foraminifera 

 which occur in the Lingula Flags of the Cambrian System. All 

 the forms described by Mr. Chapman are calcareous and vitreous, 

 such as Spirillina and various representatives of Carpenter's family, 

 the Lagenidse. 



On entering the Carboniferous System we encounter both vitreous 

 and porcellanous forms ; the latter indeed I'each their culmination 

 in the remarkable genus Fnsulina. The shell of this foraminifer 



1 ' Die Perforation cler Embryonalkammer von Penero'plis pertusus Forskal ' 

 Zool. Anz. vol. xvii (1894) p. 335 (Lit.). 



2 ' Perforation bei Peneroplis ' Archiv f. Naturgesch. 49. Jalirg. vol. i (1883) 

 p. 451. 



^ ' Foraminifera ' in E. Ray Lankester's ' Treatise on Zoology ' pt. 1 (1903) 

 fasc. 2, note on p. 95. 



* ' Lehrbnch der Paliiozoologie ' Jena, 1920, p. 46. 



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