204 FAMILY GLOBIGERINIDA. 



segment of a Globir/erina (ex, Fig. 104 *) ; the only essential difference between the two t}^pes 

 consisting in this, that each chamber of GJohigerina has a distinct communication with the 

 umbilical vestibule, whilst in Liscorbina each chamber communicates only with that which 

 precedes and follows it. The aperture is a large fissure, more or less arched, reaching to the 

 lower edge of the umbilical margin of the septal plane ; and it is nearly always more or less 

 occluded by an " astral flap," which may be a mere projection of the exogenous substance 

 deposited on the umbilicus, or may be part of the more developed " asterigerine " plates to 

 be presently described. The general form of the shell is conical, with a rather sharp margin 

 and a nearly flat base. 



352. These characters are typically shown in the Rotalia turbo of D'Orbigny, of which 

 fossil varieties had been described by Lamarck (lviii), under the names Discorbifes vesicu- 

 hiris, Hotaliles trochidiformis, and R. lenticularis. Of these the R. trocliidiformis of the 

 Grignon Tertiaries is the largest and most strongly developed, but it is scarcely the most 

 characteristic; the peculiar features of this type being more conspicuously exhibited by the 

 Rofalites vesicularis of Lamarck, which is the Rotalia Gervillii of D'Orbigny's models and the 

 Discorbina vesicularis of Parker and Rupert Jones, and which is very common at the present 

 time on the Australian coast. The form of this shell (figs. 2, 3) is plano-convex, or concavo- 

 convex, its umbilical surface being flattened, or even concave, whilst its upper surface is mode- 

 rately arched. Its segments are vesicular externally, so that its septal bands are furrowed; 

 and they commonly retain their globose form on their anterior extremity, which forms the poste- 

 rior boundary of the next segment, and is not difl'erentiated save by the loss of its pores 

 from the remainder of the chamber-wall. The upper surface may show the spire from its very 

 commencement ; but as it commonly happens that each later whorl in some degree overgrows 

 the one which precedes it, the primordial chamber is sometimes concealed. The character of 

 the upper surface is for the most part but very little altered by exogenous deposit ; though 

 sometimes the apex of the spire is crowned by a glistening boss of hyaline substance. The 

 vesicular form of the segments is sometimes retained on the lower or umbilical surface, but 

 more commonly it gives place to a flattening ; in either case, how ever, there is a deep furrow- 

 ing at each septal junction, leading towards the umbilical depression. This depression, with 

 its radiating furrows, is covered in by a sort of tent of hyaline non-tubular shell-substance, 

 which sends an astral prolongation to attach itself to the wall of each of the surrounding 

 segments (fig. 3) ; and thus a space is enclosed, which (it can scarcely be doubted) is 

 filled in the living state with sarcode-substance. Under some condition or other, this 

 " asterigerine " growth is found in almost every variety of the genus Discorbina .■ but it 

 may be reduced on the one hand to a mere rudiment, as in the B. r/lobidarls of our own 

 shores ; whilst, on the other, it may be exuberant enough to form a solid mass, as in the fossil 

 D. trochidiformis, where we find the surface of that mass covered with hemispherical granules, 

 which especially run along the margins of the septal junctions, so as to give to tiiis variety 

 a curious analogical resemblance to Rotalia Beccarii (369). 



* This very characteristic figure of the coarsely perforated Discorhina globularis is erroneously 

 given by Prof. 'Williamson as the young of tlje finely perforated Rotalina (Ptdvintilina) concamerata. 



