224 GENERAL HISTORY OF THE INFUSORIA. 



a reddish- or yellow-brown colour is ^jrodiiced. Sufficiently transparent 

 specimens and opaque fragments, \iewed by transmitted light, exliibit either 

 a glass-like (vitreous) colourless appearance, or have a brown hue. Examples 

 of the latter condition are afforded by all solid and not finely porous shells, by 

 MiUolidce, Ovulince, and others. Moreover, the youngest, thinnest, and most 

 transparent shells are rendered visible by their apparent intense brown colour. 

 Amongst porous species are some, such as Orbiculina and Sorites, which have 

 the brown colour only in stripes. Lastly, Schultze has never met with the 

 peculiar yellow, red, and violet tints mentioned by D'Orbigny in some 

 RotaUnce, Roscdince, and PlanorhuUnce. 



The figure assumed by various PoJytJudamia is extremely varied, but is 

 nevertheless reducible to certain types. We will restrict ourselves to a brief 

 description of the primary forms established by Schultze ; these are three in 

 number: — 1. In which the chambers or cells are superposed on one another 

 in a straight series. 2. In which they are disposed in a spiral manner ; 

 and, 3. in an iiTegular fashion. 



The Nodosaridoi, which have their cells placed one on another in a simple 

 row, are examples of the fii^st ty^Q ; the Sjnrocidince of the second ; and the 

 Acervulince of the third (XXI. 34). 



In spiral shells the chambers may be rolled in one plane, so as to form a 

 spnmetrical shell with opposite sides alike, e. g. in Cristellcma, or, otherwise, 

 in an asjTnmetrical mode, so as to produce a sheU. like that of the common 

 snail {Helix), e.g. Eotcdia and Bosalina (XXI. 25-28). This latter variety 

 may be so modified by the great elongation of the spiral, as to produce an 

 elongated conical outline, as in Uvigerina and Bidimina, when the chambers 

 above and below each other may present an alternate arrangement. Other 

 varieties of the spiral are exemplified in Orbicidina, Alveolina, and Nonionina. 

 In many instances a simple or regular spiral disposition is commenced in young 

 animals, which is departed from variously as they attain the adult condition 

 and characters. Thus in Planorhidina the regular s]3ii^al is transformed 

 eventually into a completely irregular form. Lastly, the Acervulince con- 

 sist of spherical or spheroidal cells aggregated into formless colonies. 



With reference to the minute structm-e of the shell. Prof. Williamson 

 {Report of British Associcition, 1855, p. 105) recognizes three principal 

 types: viz. — " 1. The hyaline, generally consisting of a transparent vitreous 

 carbonate of lime, with, usually, numerous foramina. 2. PorceUanous, white, 

 opaque, and rarely foraminated. 3. The arenaceous, mainly consisting of 

 agglomerated grains of sand." Schultze makes two tj^es : in the one, the 

 shell is perforated by numerous fine pores or canals; in the other, it is 

 homogeneous and solid. The contents of the second series are brought into 

 relation with the external world by means of one large opening, or by many 

 smaller ones collected in one group. This division corresponds, in the main, 

 with that of Prof. Williamson, except that the German naturalist has omitted 

 to notice, as a thii^d series, those shells constituted of a membrane covered by 

 extraneous particles of sand and the Hke. 



The size and distribution of the foramina, along with other stnictiu'al pecu- 

 liarities, afford the best specific characters. To examine these details the 

 shells must be view^ed by transmitted light, and by high powers. The thick- 

 waUed opaque Foraminifera are best explored, as Ehrenberg first pointed out, 

 after being soaked in some strongly refracting varnish, either entire or when 

 cut into thin sections. 



The dimensions of the canals vary in different species from -0003 of a line (a 

 scarcely measurable size) to -005 of a line. They are of extraordinary fineness 

 in Polystomella strigilata, in P. gihha. and P. venusta, whilst in Orhulina 



