GENUS MILIOLA. 81 



of the MUlola m.rormi/ mentioned \n the last paragraph, the external surface is often marked 

 by extremely deep circular pits ; but these are not usually found on the outer walls of the 

 earlier chambers of the very same specimens, and a gradational series of specimens may be 

 easily selected which should show an insensible transition from the most deeply pitted to the 

 most perfectly smooth individuals. Sometimes the shells of Miliolce, especially of the " trilo- 

 culine " and " quinqueloculine " varieties, acquire an arenaceous surface (fig. 6) from the 

 imbedding of grains of sand in the ordinary calcareous substance of the shell previously to 

 its solidification. These " arenaceous" individuals vary greatly in appearance, according to the 

 size and character of the particles of the sandy material forming the sea-bottom in their 

 respective localities. 



112. Affinities. — It is obvious from what has been already stated (^ 104), that the 

 relationship is very close between the least specialized of the " spiroloculine" forms of Miliola 

 and the ordinary Cormtspira. Again, not only do certain varietal forms of Vertebralina 

 very closely approximate Mi/iohc in their mode of growth, but Miliola occasionally takes on a 

 uniserial plan of increase resembling that of Vrrti'hraJina ; so that individuals not unfrcqucntly 

 present themselves in which the presence or absence of a " valve" is the chief diagnostic 

 character ; and even this, as we have seen (^ 109), cannot be relied on as a constant differen- 

 tiation of these two genera. With Feneroplis, again, the Milioline type is connected by a very 

 interesting modification which has received from D'Orbigny the generic designation Hauerina; 

 this designation it will be convenient for us to retain as that of a sub-genus of Miliola (with 

 which it is obviously most intimately connected), without attempting to define the precise degree 

 of relationship of the two. The real character of Hauerina was completely misapprehended 

 by D'Orbigny, who, misled by the spirality of its growth (in its early state, at least), placed 

 it (lxxiii, p. 118) among \\is HelirositY/ucs, between OjjerciiUna and Vertebralina, to the latter 

 of which it is truly allied, whilst from the former it is separated by as wide an interval 

 as can exist among any two types of Foraminifera. In the young state of Hauerina, the 

 cornu^piral form which Miliola generally present in the first instance (^ 106) is retained 

 longer than usual ; but a distinct division into chambers shows itself in the passage to adult 

 age ; and as three or (less frequently) four chambers form the circuit of the outer whorls, 

 the shell has a three- or a four-sided contour (Plate VI, figs. 34, 36). The aperture, like that 

 occasionally presented by Miliola mxornm, is cribriform (fig. 36, b).* Although described by 

 M. D'Orbigny as only existing in a fossil state, this type occurs at the present time in the 

 Indian, Australian, and other tropical and sub-tropical seas. The external surface of its 

 shell is sometimes smooth, sometimes delicately striated (fig. 36, a) ; whilst sometimes its 

 walls are very deeply plicated transversely (fig. 35), — a varietal modification of which 

 we have already seen a less strongly marked example among the ordinary Miliohe (fig. 5). 



113. By the Miliola saxoriun, again, the Milioline type is obviously brought into close 



* In the description and figure of H. compressa, given by D'Orbigny (lxxiii, p. 119, pi. v, 

 figs. 26, 27), the septal plane is represented as perforated by a single small, oval aperture, surrounded 

 by several large, granular elevations ; but these (as appears from the examination of recent specimens 

 of the same type) really mark the place of passages which have been filled by fossilization. 



11 



