92 FAMILY MILIOLIDA 



can scarcely fail to perceive how slight are the differential characters which separate it from 

 either Vertehralina or Hauerina. On the other hand, it presents a very close approximation 

 to the spiral variety of Orbiculina ; a sort of preparation for the subdivision of the chambers 

 characteristic of that type being made in the subdivision of the aperture, and perhaps also 

 in the ridge-and-furrow arrangement of the lateral walls of the chambers. And, as we shall 

 hereafter see {% 143), there are certain aberrant forms of Orbiculina in which the subdivision 

 of the chambers is wanting, so that no perfectly defined boundary between these two types 

 can be said to exist. 



132. Geographical Distribution. — The genus Peneroplis is very widely diffused through 

 warmer latitudes, especially frequenting the laminarian zone ; so that few collections of Fora- 

 minifera from sands or dredgings brought from the Mediterranean, the ^gean Archi- 

 pelago, the Red Sea, the East or West Indies, the Philippine Seas, or the shores of Australia 

 or the Polynesian Islands, will fail to present numerous examples of it. A few specimens 

 have been found on British coasts ; but it seems most probable that, as Prof. Williamson has 

 suggested (ex, p. 46), these have been brought by the Gulf Stream from the West Indian 

 Seas. The prevalence of particular modifications seems in some degree determined by 

 temperature. For it is only in tropical seas that the Dendritine variety presents itself 

 127); and although the ordinary type also abounds in the same localities, yet it is there 

 that it shows the greatest tendency towards the dendritine variety, both in the turgidity of 

 its spire and in the arrangement of its septal pores, whilst the Spiroline variety generally 

 shows some tendency to the dendritic form of aperture. In the Red Sea the ordinary 

 Peneroplis is very abundant ; and in its young state the spire is turgid, and its pores are 

 frequently arranged in a double row or even tend to coalesce, though with each addition to 

 the number of segments the spire becomes flatter and broader, and in the older specimens 

 the pores are almost invariably arranged in a single row. Now here the proper Dendritine 

 variety is either absent altogether, or is extremely rare ; and the Spiroline (which presents 

 every gradation from the cylindrical to the compressed form) rarely has any other kind of 

 aperture than a multiplication of separate pores. In the Mediterranean, on the other hand, 

 which seems to be the most northern limit of its diffusion, not only is the Bendritine variety 

 altogether wanting, but the Peneroplis type seems (as it were) to be starved out, the spire 

 presenting the extreme of attenuation, and the septal pores being almost uniformly arranged 

 from the beginning in a single row. — The Spiroline variety, which is usually of smaller 

 dimensions, seems to replace both the Dendritine and the Peneroplis type in the deeper waters 

 of the tropical ocean ; both the large turgid Bendritina and the broad Peneroplides being 

 inhabitants of comparatively shallow water. 



133. Geoloyieal Bistribution. — Neither the ordinary Peneroplis nor its Dendritine and 

 Spirolme varieties has been traced backwards in geological time further than the commence- 

 ment of the Tertiary epoch.* The Grignon and Paris Tertiaries contain several forms of 



* The statement of some authors that Spirolina occurs in the Cretaceous and even in earlier 

 formations of the Secondary epoch, has resulted from their having mistaken the crozier-shaped forms 

 of Lituola for the true Spirolina. 



