GENUS PULVINULINA. 211 



366. The characteristic form of PuJvimdina is subject in the Laminarian zone of warmer 

 latitudes to a modification so remarkable, that its Rotaline character would scarcely be recog- 

 nisable if its affinity were not traceable through intermediate gradations. This form (Plate 

 XIII, figs. 4 — 6) has been described by D'Orbigny under the designation PlanorbuUna 

 vermiculata ; but although to a certain extent analogous to PlanorbuUna in its plan of 

 growth, it differs from it entirely in other characters, and can be readily shown to be essen- 

 tially a PidvinuJina extremely flattened-out, of which the later chambers elongate themselves 

 like the continuous tube of a Spirillina, with very few interposed septa, sometimes returning 

 into themselves so as to form complete though irregular annuH, of which the earlier are 

 surrounded by the later. The substance of the shell in this curious variety is very finely 

 porous, the pores being grouped in little bundles (fig. 6), and sometimes looks almost horny 

 in its transparency and hue ; but on its attached surface (fig. 5) we notice that it is (as it 

 were) punched-through with large irregularly scattered apertures. These expanded forms 

 sometimes attain a diameter of l-7th of an inch j on the other hand, the contracted 

 deep-sea forms have no greater a diameter than 1-2 50th of an inch. The diameter of the 

 ordinary trochoid forms of moderately deep water ranges from about 1 -20th to 1 -50th of an 

 inch. 



367. Affinities. — This genus holds among Rotalines, in regard to the fineness of the 

 texture of its shell, and the freedom of its mode of growth, a rank analogous to that of 

 Bdimi/ia in the Textularine series. Its nearest affinity, however, out of its own sub-family, is 

 to SpiriUina, to which its lowest forms approximate closely in mode of growth ; whilst the large 

 pores that are met with in the chamber-walls, not only in these but in higher forms, mark the 

 persistence of an Orbuline condition of the animal. In this genus, moreover, as in the two 

 preceding, we find complanate nearly symmetrical varieties with raised edges, that bear a 

 strong resemblance to OpercidincB. 



368. Geographical and Geolocjiral Bistrihdion. — The conditions under which this genus 

 flourishes best are by no means the same as those which most favour the development of the 

 two preceding types ; for some of the largest examples of Pidvinulina come to us from within 

 the Arctic Circle, and plenty of good-sized specimens may be obtained from extreme depths. 

 The examples which occur in the Algal zone, on the other hand, are for the most part the 

 feeblest. It is, perhaps, on account of its prevalence at greater depths, that this type has 

 been distinctly recognised at an earlier geological period than any other of the Rotalines ; the 

 P. elegans having been found so abundantly in the Upper Triassic clay of Chellaston by 

 Messrs. Rupert Jones and Parker (lv), as to constitute nearly one-half of the Foraminifera in 

 which that deposit is rich. The genus continues to present itself in the Liassic and Oolitic 

 series, its examples being generally small but strongly limbate ; in the Gault their size 

 increases, the limbation being still very strong ; and they become still larger in the Chalk- 

 marl. It is very probable that these diversities have reference only to the depths at which 

 the clay-beds that have been hitherto examined in these formations were respectively depo- 

 sited. The highest development of this genus seems to have been in the Tertiary series, and 

 especially in the Crag-formation. 



