GENUS CASSIDULINA. 197 



to a tri-serial or bi-serial alternation, and that in unquestionable Textularim the bi-serial 

 arrangement may give place to the tri-serial or even to the bulimine ; whilst through all these 

 variations the form of the aperture characteristic of each type is constantly maintained. 

 The relation of the large sandy BuUmince to the "bulimine" Valmdina (^221) is extremely 

 intimate ; the form of the segments and their mode of aggregation being the same ; the 

 originally porous texture of the shell being common to both, and being liable in both to be 

 afterwards more or less completely occluded by arenaceous aggregation ; and the difference of 

 type being marked only by the aperture. The feebler forms of BuUmina are related, like those 

 oi TcvtuJarina, to Globit/erina ; and in some of the loosely piled tri-serial Bulimi/ice there is a 

 strong general resemblance to certain JJvigerince, the chief differentiation being in the aperture. 

 On the other hand, in BoJivina punctata, which combines a crescentic curvature of the entire 

 shell with a bi-serial arrangement of the segments and a very obhque aperture, we have an 

 obvious tendency towards Cassidulina, in which form the series appears to culminate. 



335. Geo(/raj)ldcal Distribuiion. — There seems to be a close correspondence between 

 BicUmina and TextuJaria as to the conditions most favorable to their development, as they 

 are generally found under the same circumstances ; the latter, however, being commonly 

 much more abundant than the former, 



336. Geological Distribution. — This genus has not as yet been traced, however, quite so far 

 back in geological time as Textularia, its earliest appearance, so far as we at present know, 

 having been in the Upper Trias, in which small examples of it are found ; it occurs in the Oolitic 

 formation and in the Chalk-marl, in which last deposit and in the Chalk some of its finest 

 examples present themselves. It occurs under some or other of its forms through the whole 

 Tertiary series, from the London Clay to the most recent formations ; some of its most remark- 

 able varieties being found in the Vienna and San Domingo deposits. 



(?e«K5 XII.— Cassidulina (Plate XII, fig. 23; and Williamson, Figs. 141—144). 



337. History. — This genus was first established by M. D'Orbigny in 1826 (lxix) for a 

 peculiar type, which does not seem to have been recognised by any previous observer. 



338. External Characters and Internal Structure. — This genus is distinguished from all 

 other Foraminifera in combining the bi-serial and the convolute arrangement of its segments ; 

 for although several other genera are associated with it by M. D'Orbigny in his order Ento- 

 niosti'f/ues, yet in no one of them (as will hereafter appear) does this combination really exist. 

 The texture of the shell of Cassidulina is hyaline and finely porous, like that of the smaller 

 Bulimiiice ; and it resembles that type also in the characters of its aperture. In fact, if we 

 imagine a bi-serial BuUmina to be completely rolled upon itself, so as to form an equilateral or 

 nearly equilateral spire, we should have the essential features of a Cassidulina. The arrange- 

 ment of the chambers, as it shows itself externally, has a semblance of irregularity which does 

 not really belong to it; the apparent irregularity being really due to the interdigitation of the 



