GENUS BULIMINA. 195 



porous structure is considerably finer than \.\nioi Tcainhirin; the hyaline surface is never lost 

 in the recent examples of this type ; but the larger and older fossil specimens very commonly 

 have the shell incrusted by an adherent layer of arenaceous particles. Such arenaceous 

 forms are very common among the Bulimince of the Cretaceous deposit, of which an example 

 is here figured (Plate XII, fig. 18). In the BuJimince, as in the Ttwtidarite, the aggregation of 

 segments takes place upon the simple plan which is common to the lower forms of the vitreous 

 series with the whole of the porcellanous ; the anterior wall of each chamber serving as the pos- 

 terior wall of that which succeeds it, and the septal laminee being single and not differentiated 

 from the ordinary wall of the chamber. There is therefore neither intermediate skeleton nor 

 canal-system. 



3-n. Departures of greater or less extent from the typical character of this genus are 

 extremely common ; and such departures are especially liable to occur in regard to the plan 

 of growth, to the form and setting-on of the chambers, and to their surface-ornamentation. 

 Thus the obliquely spiral succession of the segments may be so modified that they come to 

 present a more or less distinct tri-serial arrangement, ofwhich wehave a striking illustration in 

 the B. Biichiana of D'Orbigny (Plate XII, fig. 19), whilst the ordinary B.pupoldcs (ex. Figs. 

 124, 125) presents it in a less obvious degree; and in a variety of this last form termed 

 fusi/onins by Prof. Williamson, the segments are chiefly added in /m>o instead of in three oblique 

 series, so that the form becomes compressed (ex. Figs. 129, 130). A still greater modification 

 is effected by the uncoiling of the spire, so that the later segments present a uni-serial recti- 

 lineal succession, and the form of the entire shell becomes " spirohne ;" of this we have an 

 example in the varietal form described by Prof. AVilliamson as coiivolida (ex, p. 6.'}, Figs. 

 132, 133), of which he remarks that "it presents almost as many modifications as individual 

 specimens."* The same condition sometimes presents itself in the B. elef/antif<f:iiua of the 

 Indian seas ; which also sometimes puts forth a few feeble " wild" segments out of the regular 

 axis of growth. In other varieties, again, the ventricose form of the segments is modified in 

 a greater or less degree by flattening or by drawing out in some particular direction. Thus 

 the earlier segments may be more or less completely enclosed by the backward extension of 

 the later. Of such backward extension we have an incipient example m B. marcjinata (xc, 

 Figs. 12G, 127), but it is carried to its fullest extent in B.p>jrida (Plate XII, fig. 20), in which 

 the later segments so completely enclose the earlier, that only the last three are at any time visible. 

 By a lateral elongation of the segments, the obliquity of their setting in relation to the axis 

 of the spire, and the flattening of their borders one against the other, that peculiar varietal 

 form is produced which has been distinguished by M. D'Orbigny under the generic desig- 

 nation Bobertina (Plate XII, fig. 21) ; and it is a singular instance of that want of power to 

 discern the real aflinities of the types he described, which I believe to have been induced by 

 his misplaced confidence in plan of growth as the only trustworthy guide in the 

 classification of Foraminifera, that he has not only separated this well-marked Bulimine type 



* This form is veganlcd by Prof. AVilliarason as a 4(-serial -B. piipoides partly rolled upon itself ; 

 but he seems to us to have been misled by the split appearance which the shell acquires from the suc- 

 cession of the fissured apertures of a ufii-seiial row of segments ; and we cousider it, as above stated, 

 rather as an uncoiled than as an abuormallv convoluted form. 



