206 FAMILY GLOBIGERINIDA. 



said to be universally diffused, and to have a very wide bathyraetrical as well as climatic 

 range. But its several generic forms are by no means equally developed under different 

 conditions. Thus Discorbina, though presenting itself on the British coasts, in the Mediterra- 

 nean, and in the East India seas, is there comparatively small ; attaining a much larger size 

 near the Australian shores. Its most congenial home seems to be in the Algal and Coralline 

 zone, the deep-water examples of this type being smaller and rarer ; a sandy bottom seems to 

 suit it best, the specimens found in mud being small and delicate. — These peculiarities of 

 habit must be taken into account in estimating the geological range of this genus; for it is 

 quite possible that its absence from all strata in which it has been searched for, earlier than the 

 Maestricht beds, may be due to the fact that these strata were deposited for the most part in 

 deep seas. In the Maestricht beds this genus is represented by specimens of medium size ; 

 its largest fossil forms (which equal or even exceed those found recent on the Australian 

 coast) are furnished by the Bracklesham and Grignon deposits ; whilst in the later Ter- 

 tiaries its varieties cloely correspond in size and character with the ordinary existing forms of 

 the genus. 



Ge;;«s XIV.— Planorbulina (Plate XIII, figs. 13—15; and Williamson, Figs. 119—123). 



357. External Characters and Internal Structure. — The modification of the ordinary Rotaline 

 type of which we have next to treat, is one that departs more widely from it in plan of growth 

 than it does in the character of the individual segments ; these retaining very much of their 

 simple Globigerine condition, but being disposed in such a manner that their originally 

 continuous spiral succession gives place to a cyclical arrangement not unlike that of Orbitolites, 

 whilst this again may give place to an irregular heaping-up or " acervuline" disposition of the 

 segments. It will be convenient in the first instance to describe a characteristic example of 

 this type, and then to examine on the one hand the links which connect it with the ordinary 

 Botulines, and on the other those (so to speak) exaggerated forms by which it comes into 

 relation with the genera Tlnoporus and Poli/trcma. 



358. Such a characteristic example is presented by the Planorhulina mediterranensis 

 of our own coasts (described and figured by Prof. Williamson, ex, p. 57, Figs. 119, 120), 

 which attains a considerably larger size in the Mediterranean and in other sub-tropical 

 and tropical seas, and which has also received the name of P. farcta. The growth 

 of this shell, which is always attached on one side, commences in a very depressed 

 rotaline spiral ; the successive segments of which open into one another by a wide 

 fissure that is close to the inferior lateral surface, which (by the inverted position of the 

 shell, ^ 345) is here its free surface. After the spiral has made from one and a half 

 to two turns, however, the segments begin to be added in a manner altogether different ; 

 the change being made in the first instance by the formation of segments which bud forth 

 new ones from their peripheral as well as from their umbilical margins ; and from 

 these a succession of segments is produced, which passes round the original spire, and then 

 returns into itself. Each of the chambers thus formed has an oval opening at either extremity. 



