GENUS SPH^ROIDINA. 185 



climates ; its occurrence being rarer and its examples smaller in shallower waters, and 

 in the seas of northern temperate or arctic regions. It makes its first appearance in the 

 Chalk-marl, and has been recognised also in the London Clay ; but it is most abundant in 

 the Vienna, Grignon, Sienna, and other Tertiary deposits. 



Genus VI. — Sph^roidina (Plate XII, fig. 13). 



303. Hidory. — The first account of the very peculiar type known under the name of 

 Sjjharoidina seems to have been given by M. D'Orbigny in 1 825 (lxix) ; but he confesses himself 

 to have so far misunderstood its plan of growth, as to have erroneously placed it among his 

 Enallostiijues. Subsequently (lxxiii) he was induced to place it among his Jr/afliisfer/ues, in 

 immediate association with the Milioline series, to which its relationship, according to our 

 view, is analogical only. 



304. External Characters and Internal Structi(re. — The shell of Spliaroidina is most dis- 

 tinctly vitreous and perforated ; but its parous texture is finer than that of Glohigerina. It 

 consists of a series of segments which succeed one another in a loosely turbinoid spire, having 

 but four or five chambers in each turn. These segments are turgid and vesicular, like the 

 chambers of Glohigerina ; but each invests a large part of the preceding, so that only three 

 segments are usually visible (Plate XII, fig. 13). The aperture is generally more or less 

 crescentic in form, and is situated at one side of the last chamber close to the exterior of the 

 oldest visible segment ; it is partly occupied by a projecting tongue or valve, which strongly 

 resembles that of Miliola ; and while its form sometimes approaches that of a Spirolocidina 

 (Plate VI, fig. 18), it as frequently approaches that of a BUoculina (Plate VI, fig. 7). Some- 

 times its margin has deep radiating notches. — A curious variety is presented by certain examples 

 of Sphceroidina, whose chambers, instead of being in close contact with one another, are 

 separated like the segments of a dehiscent fruit ; the intervals, however, being partly bridged 

 over by shelly bands. 



305. Affinities. — It will be obvious, from what has now been stated, that both in the 

 structure of the individual segments, and in the general mode in which they are arranged, 

 there is a very close resemblance between Sphceroidina and Globigerina ; the difference between 

 these types chiefly consisting in the continuous communication of the chambers of the former, 

 one with another, and in its valvular aperture, as also in the degree in which its earlier 

 segments are overgrown by its later. There is no real correspondence in plan of growth 

 between Spharoiditia and Miliola ; so that the only point of resemblance between them con- 

 sists in the valvular aperture that is common to both. 



306. Geographical and Geological Distribution. — It is not a little confirmatory of the 

 correctness of the place we have assigned to this type, that its habitat should remarkably 

 correspond with that of Orhulina, Globigerina, and Pidlenia ; the home of these four types 

 being at depths in the ocean at which other Foraminifera are comparatively rare. It is 

 lound especially, but not exclusively, in the seas of the warmer regions of the globe. — The first 



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