48 OF THE FORAMINIFERA GENERALLY: 



61. To separate all the Foraminifera that form "arenaceous" shells from tliose of the 

 " porcellanous " and " hyaline " t3'pes, to which many of them obviously bear the closest 

 affinity, would be a violation of the first principles of a natural arrangement ; and yet we 

 shall find that there are certain generic types in which the sandy texture is a character of 

 great systematic importance. Thus, on the one hand, in the low " porcellanous " type Nuhc- 

 cularia, and in the "triloculine" and " quinqueloculine" forms of the more elevated type Miliola, 

 individuals frequently present themselves whose surface is rendered arenaceous by the im- 

 bedding of sandy particles in their ordinary calcareous shell-substance ; but for being thug 

 " rough-cast," such individuals would present the ordinar}' aspect of their i-espective types, to 

 which they are entirely conformable in every other character ; and it would be manifestly 

 improper to rank them apart on account of this trifling variation. So even when we find 

 truly arenaceous shells presenting the characteristic forms of the " hyaline " genera BuUmina 

 and Textularia, since we find at the same time that not only do they correspond in general 

 structure to the calcareous shells respectively peculiar to those genera, but are perforated like 

 them by regular pores for the passage of pseudopodia, we feel that we have no right to dis- 

 sociate what are manifestly nothing else than varieties of a common type. But when, on 

 the other hand, we find that certain assemblages of forms, constituting well-marked generic 

 types, can be uniformly characterised by the possession of "arenaceous" shells, — as is the 

 case with Trochammina, ValviiUna, and Liiuola, — it becomes obvious that this peculiarity is to 

 be regarded as a distinctive feature of higher value, since it marks a fixed and decided phy- 

 siological condition, the occurrence of which elsewhere is only occasional or incomplete. The 

 absence of any pseudopodial pores in the walls of the chambers of the shells of this group shows 

 their affinity to be rather with the porcellanous than with the hyaline series, notwithstanding 

 the very close resemblance in form which some of them present to particular types of the 

 latter, — Valvulina, for example, to BuUmina, certain Lifuola to Nonionina, and TrocJiammina 

 to SpirU/ina. 



62. Turning now from the ultimate texture of the shell to its conformation, we have to 

 inquire whether there are any fundamental and essential diversities in its mode of increase, 

 which can be advantageously used as difl'erential characters. It may be stated as an un- 

 questionable fact, that the shelly casing, once formed to any portion of the sarcode-body, cannot 

 be enlarged by interstitial growth ; and hence it can only be adapted to the augmenting 

 dimensions of that body, by addition to the part already formed. In the greater part of the 



Fig. II. 



sometimes united in series, after the manner of certain forms of 



Foraminifera ; thus in Fig. II, i, is shown a Nodosarian series of four 



equal cells ; whilst at ;; is seen a Textularian series of seven cells, 



which increase regularly in diameter from l-1350th to l-450th of an inch. 



The " coccoliths " do not appear to be attached to the wall of the " cocco- 



sphere" in any other way than by gelatinous adhesion ; and hence they 



are very easily detached, so as to form a considerable proportion of 



Coccosphcies, with attached m^ny deep-sea deposits. Similar bodies have been found in Chalk by Mr. 



coccoliths, growins; (1) on 



the Nodosarian, (2) on the ^" ^°''^y- 



Textularian type. 



