GENUS ROTALIA. 213 



the umbilicus open, as in the recent feeble forms. In the strongest tropical forms of moderately 

 shallow water, on the other liand, of which the R. Sckroeteriana (erroneously designated 

 Faujasina by Prof. Williamson cix) is a characteristic example, the form of tiie shell is com- 

 pletely changed, by the peculiar form and disposition of the segments, from that of a Turbo 

 to that of a Conus ; the upper surface being now flat or nearly so, whilst the lower surface 

 becomes conical (Plate XIII, fig. 7). Notwithstanding this striking metamorphosis, the 

 essential identity of the two types is proved by the cxistenL^. of a complete series of grada- 

 tional links between them, some of these intermediate forms being almost equally conical 

 on the two sides, but showing the whole spire on one lateral face, whilst only the last 

 convolution is seen on the other. In the typical R. Sckroeteriana, which attains a 

 diameter of l-12th of an inch, the upper surface is rendered " limbate" by the prominence 

 of the septal bands, which are ornamented with rows of hyaline tubercles; and these tubercles 

 are continued along the septal bands down the side of the cone, until they join a large irre- 

 gular mass of the same semicrystalline substance tliat occupies the apex of the cone, which is 

 here tlie real umbilicus of the spire. Between these tubercles there may be distinguished, under 

 a sufficient magnifying power, a single or double row of large pores along each septal band. 

 The aperture has more or less of an oval form, and is usually situated about half-way between 

 the upper flat surface and the umbilical prominence. In a remarkable variety of this type 

 from the Fiji islands, the spire is more symmetrical, and the aperture is bridged over by trans- 

 verse bars (as in Calcariiia), so that the shell becomes almost isomorphic with Pohjdomella 

 craticidata ; to which type it further bears considerable resemblance in the disposition of the 

 exogenous deposit, and in the mode in which the intraseptal passages open on the surface 

 along the septal bands (§ 372.) 



371. Various weaker modifications of the preceding forms present themselves at different 

 depths in the tropical ocean and in marine deposits of different geological ages. Thus, from 

 100 fathoms and more we have recent examples of the R. Soldanii of the Vienna Tertiaries 

 (lxxiii), which is identical with the R. lunhilicata of the Chalk ; a close thickset smooth 

 form, without septal granulations, and having its umbilicus sometimes occupied by a single 

 large rounded tubercle. Again the R. orbicularis (which ranks as GyroidiiKi in D'Orbigny's 

 models) is almost hemispherical in form, its chambers being quite " flush," and its upper flat 

 surface showing the entire series, which consists of about forty segments, whilst its lower 

 spherical surface shows ten or twelve. A still more marked departure from the typical form 

 is produced by the extension of three or four of the septal bands of the first convolution into 

 long smooth radii, giving rise to the variety described by D'Orbigny as Cah-uriaa jjitlchella, 

 which is further remarkable as having each of its raised septal bands double. And finally, 

 in a conus-shaped tropical variet)', which has received from D'Orbigny the names Antcrii/criiui 

 carinala and A. lobafa, the umbilical lobes of the chambers are separated from the principals 

 by the intervention of a septum, so as to constitute a secondary series of chamberlcts which 

 interdigitate or dovetail themselves between the proper chambers, as do those of the lower 

 surface of Ampliistefjina vuhjuris and its varieties (Plate XIII, fig. 23), instead of lying in a 

 lower plane like the subsidiary chambers formed by astral flaps in Divcorbiiia (^ 352). 



372. Internal Structure. — The internal structure of the most developed form of the genus 



