GENUS PLANORBULINA. 209 



growth. Sometimes the segments do not come into absolute contact with each other, being 

 kept somewhat apart by the intervention of the apertural necks ; and each chamber then has 

 its own complete walls. This varietal modification, however, is much more remarkable in a 

 ])!irasitic form, which has received from Messrs. Parker and Rupert Jones the name of 

 P. retinaculata ; for the segments are here smaller, whilst the connecting tubuli are relatively 

 larger and more numerous ; so that the disk is composed of a sort of open network that 

 spreads over the surface of the shell whereon it grows. Both in this and in the large varieties 

 of P. vulgaris, the free surface is studded over with large tubercles which arise from it be- 

 tween the large and sparsely-set pores ; these tubercles are sometimes formed of a remarkably 

 white opaque kind of shell-substance, giving a very peculiar aspect to the organism (Plate 

 XII, fig. 15). The Acervulina of Schultzc (.xcvit, p. 67) is nothing else than a Planorbiduia 

 which attaches itself to the stem of a Zoophyte or some other small rounded body, and of 

 whicli the segments, instead of spreading discoidally over an expanded surface, cluster 

 together in a mass, with more or less of regularity of arrangement. Sometimes the mass 

 acquires considerable compactness from the mutual flattening of the segments against each 

 other ; and the " acervuline " PlanorbuUiia may thus come to present so close a resemblance 

 to Tiiiojmrm and Polytremu, as to be almost undistinguishable from those types. 



362. JffinUk's. — Besides its affinities to the ordinary Rotalines, and, as we have just seen 

 to Tinoporus and Polytrema, this genus is related by some of its most symmetrical " anoma- 

 line" forms to the simpler nautiloid types of the family Ni/mmulinida ; whilst its arrested 

 " truncatuline" forms are closely paralleled in general configuration by the " placopsiline" 

 LitiioJcE (t 213). In the cyclical arrangement of its segments of growth, which results from 

 the mode of increase that may be considered typical of its advanced stage, Plaiwrbulina seems 

 to conduct us towards PatcUma ; in which we shall find the parallelism to Orbitoliteti most 

 remarkably carried out. 



363. Geographkcd and Geological Distribution. — The conditions which most favour the 

 production of P/rt//Of(^«///^« seem to be almost exactly the same as those under which i)/.wo?-5///a 

 most abounds. The numerous specimens which come to us from arctic and temperate seas 

 are of small size and arrested development ; in both respects the Mediterranean forms of this 

 genus show a decided advance ; but it is in the tropical and Australian seas that it finds its 

 congenial home amongst sea-weeds, corallines, and shell-beds, the specimens found at abyssal 

 depths (1000 fathoms and more) being small and rare.— With respect to the apparent absence 

 of this genus in the earlier stratified deposits, the remark already made in regard to Discorbiiia 

 (t 356) equally holds good here ; and it is interesting to observe that although Plaiiorhuliiia 

 makes its appearance in the Lias, it is represented there by very small forms, such as at 

 present inhabit great depths ; and that the like condition prevails for the most part in the 

 Cretaceous formation. In the Tertiary series, however, especially in the Grignon deposits, it 

 is often very large, although arrested in its development, its forms being identical with such 

 as now present themselves at depths of from 30 to 100 fathoms. 



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