GENUS POLYSTOMELLA. 287 



484. J^ni/ies.— Although the typical Pohjsiomdla differs so remarkably from any other 

 known form of Foraminifera, yet in those varieties in which its peculiarities are (so to speak) 

 softened down, we trace an obvious affinity to the ordinary Operculine type. Thus in P. nixpa 

 the reduction of the umbilical deposit to what is little more than the large tubercle often 

 presented by Operculina, the lenticular form of the shell as displayed in a vertical section, and 

 tlie limited distribution of the canal-system, constitute one set of hnks; whilst in tlie P. slrialo- 

 jy«;/e/'«fo another set is established in the suppression of the crenulations for the lodgment of the 

 retral processes, and in the conversion of the cribriform aperture into a continuous slit. Thus 

 we may regard Poli/sfoiiicUa as a sort of offset from the Nummuline series ; distinguished, in 

 its highest evolution, by the extraordinary and very regular development of its canal-system, 

 by the crenulations for the lodgment of the retral processes of the segments, by the excessive 

 amount and peculiar disposition of the exogenous deposit which forms its supplemental skele- 

 ton, and by the substitution of a row of separate pores for the single continuous fissure. If, 

 however, we carefully examine into the value of any of these characters as marking an abso- 

 lute difference of type, we find that they are all gradational, and are in no instance all com- 

 bined in the highest degree. Thus the approximation of the canal-system in P. crlsjjn to the 

 ordinary Operculine type makes it evident that the meridional canals which are so remarkable 

 in P. craticiilata and P. sfriato-pmdafa are really nothing else than the intra-septal arches of 

 the canal-system in Opcnul'ma (^ 445) ; and that the rows of diverging branches of the 

 former correspond with the extension of the intraseptal system into the marginal cord of the 

 latter, its peculiar modification being related to the marked difference in the disposition ot 

 the layer of exogenous substance, which in the one case completely invests the external 

 surface of the shell, whilst in the other it is limited to a narrow marginal band. Again, the 

 retral processes which constitute the" most distinctive feature of the animal in P. crispa, and 

 are lodged, not only in grooves but in tubes of its shell, are far less developed in P. craficitlata, 

 and are almost or completely suppressed in P. ninato-puHctata. The exogenous deposit, which 

 is generally abundant in P. cratiailata, is sometimes present in a comparatively small amount 

 both in that and in other forms ; so that its presence or absence affords no proof of essen- 

 tial distinctness. And the cribriform aperture gives place in the Nonionine modification of 

 this type to the ordinary Operculine fissure. There is nothing, therefore, to forbid the idea 

 that this diverging form may have been the result of gradual modification. 



485. Geographical Distribution.— This genus has a world-wide range, representatives of 

 it being found in all seas. The predominant form in the tropical ocean seems to be P. 

 craticuluta ; in temperate seas it is P. crispa ; and in the arctic zone it is P. striato-piniclata. 

 The Nonionine sub-type seems to prevail especially in temperate latitudes and in shallower 

 seas than those frequented by the typical Polystomella. 



486. Geological Bistriljulion. — The only example of the typical Polystoniclla that has 

 yet presented itself earher than the Tertiary period is that described by D'Orbigny 

 (lxxiii) under the name Fuujasina carinatu (^ 482), which occurs in the Upper Chalk of 

 Maestricht. With the Eocene period, however, this generic type seems to have become 

 more widely diffused ; as it occurs in the early Tertiary strata of various parts of the world, and 

 abounds in many deposits of the middle and later Tertiary periods, its examples being espe- 



