GENUS DISCORBINA. 205 



353. This type keeps very strictly, as a rule, to its regular mode of increase ; though 

 sometimes iu the larger examples of the B. vesicularis we find the spire terminating in a few 

 irregular or wildly-growing chambers. These may be either unusually large, or may become 

 suddenly small ; and they may either intercalate themselves along the border of the preceding 

 segments, or they may grow over the older part of the shell without any definite direction. 

 The smallest and feeblest varieties of Discorbina form a high cone, which may be flattened 

 on two surfaces, so as to present a very " textularian " aspect ; on the other hand, the spire 

 may be so much depressed that the shell comes to have a complanate or even a scale-like form. 

 It is for the most part in the small complanate varieties that we find an elevation of the 

 septal bands and of the margin of the spire, which, common as it is in Pidvinulmn, is of rare 

 occurrence in this group ; of this an example is shown in the Rofaluia ocltracea of Williamson 

 (ex. Figs. 1 12, 113). One of the most curious modifications of structure in this type consists 

 in the existence of deep fissures between the successive segments, the walls of which are 

 complete on either side of these fissures ; as we see in many recent examples of the D. vesicu- 

 laris of Australia, but still better in the D. rimosa (Parker and Rupert Jones) of the Grignon 

 Tertiaries and the Australian coral reefs. In this variety we not only find the asterigerine 

 flaps so much developed as to form a regular series of secondary chambers alternating with 

 the primary, but we also find the exogenous deposit partially bridging over the superficial 

 entrance to the interseptal fissures, dividing it into a row of little passages ; and thus is pro- 

 duced a sort of sketch of that intraseptal system of canals, which we shall see to be more 

 fully elaborated in Botalia, and to attain its most complete development in Polystonella. 



354. Although, as a general rule, the walls of the chambers are coarsely perforated, 

 yet in some of the smaller and more delicate varieties the pores are very fine. There are 

 some instances in which a considerable portion of the external surface of each segment ia 

 destitute of pores, these being limited to the margin, where they are unusually large and 

 closely set, giving rise to the peculiar appearance represented by Prof Williamson (ex, Figs. 

 109, 111) as characterising his Hotalina mawilla, a variety of the Rofalia rosacea, D'Orb. In 

 some varieties these marginal pores are prolonged into short tubes, which give to the edge a 

 prickly character. The diameter of the shells of this genus ranges from l-6th [D. trockidi- 



formis) and l-7th {D. vesicularis) of an inch to as little as l-200th of an inch, which is the 

 diameter of some of the specimens brought up from the deeper recesses of the ocean. 



355. Affinities. — It has been already remarked (^ 341) that all the genera constituting 

 the proper Bofaline series are intimately connected by mutual affinity; but each of them has 

 also relations of its own to other types. Thus, as we have seen, the simplest forms oiDiscorbina 

 connect themselves with Glohir/erina, and even with Textularia ; whilst among its more deve- 

 loped forms we find approximations to the lower types of the family Nuiiimulinida, the com- 

 planate varieties reminding us of Operculina, and those in which the " astral lobes' are most 

 complete bringing us almost into contact with Amphistegina. In general configuration this 

 genus is closely paralleled by certain forms of the protean arenaceous type Littiola 

 {\ 213). 



356. Geographical and Geological Distribution. — The Rotaline group as a whole may be 



