71 



FORAMIN1FERA. 



FOUAMINIFERA. 



871 



menu is wound round the primary central one, u the thread is around 

 a Iwll of wonted. (Qxim/ndociUtaa, Triloculina.) 



In the preat majority of the specie* the interior of each chamber is 

 siraplo and undivided, but there are Rome forma, especially amongtit 

 the Jfrlicnitrga, in which the newer and more external chamber* are 

 subdivided either by transrerae pillars or by complete partition! 

 perforated by one or more a|>ertures. through which prolongations of 

 the gelatinous Rubtance unite the various segment* of the soft animal. 

 (OroicWmo, jiy. 11.) Ordinarily but one such chain of communica- 

 tion* exit (animal of Rotaii*a, fg. 12); but in the cases just referred 

 to, there u a great increase in the number of such orifices, so that 

 the septa become completely cribriform. The distribution of these 

 aperture* affects the gemmation or mode of growth, since it is 

 through them that the new segment* nre successively formed, the 

 gelatinous substance being extended by a process of budding or 

 sprouting. An increase in the number of such orifices is most 

 common where the consecutive segments present a rapid increase in 

 their size. In the genus Orliic*lina, this growth is sometimes fo 

 remarkable that the new segments soon form concentric circles, 

 embracing all those previously formed. (Orbiculina complanata, fig \\ ,) 

 In such examples the connecting apertures are distributed round the 

 entire periphery, and gemmation most probably takes place simulta- 

 neously through them all ; so that the soft animal, if decalcified by an 

 acid, would present a succession of rings, inclosing one another, and 

 connected together by transverse radiating bars. 



The memoirs of Professor Williamson have shown that the shell 

 inclosing each new segment in at first very thin ; but as additional 

 calcareous chambers are formed, each such addition not only encases 

 the new gemmation of the soft animal, but extends over all the 

 exterior of the previously-formed shell. The exact way in which this 

 is accomplished is doubtful ; but it is probable that the soft animal 

 hm the power of diffusing itself over the shell, and depositing upon 

 its surface additional layers of calcareous matter. 



18 



. Omuidulina. 9. Ttzlilaria. 10. Spiral orul in*. II. Orticulina com. 

 planala. I la. Part of two chamber* of an OrUeulina. 12. Decalcified 

 nlml of I Rfntina. IS. Ratal ina ftolularii, Ti*wrd *l * truupamit objrct 

 bowing the foramina. 14. Horizontal section of Fig. 6, ihowinu the mtrrni! 

 rtm of tube*. IS. Powil Xummulma. 16. Vertical Mellon of Fig. 11. 

 l;. I'mirunliiM Irirariaola : from the Chalk. 18. Rotalina Lornnaria, 

 from Uw Cb*lk. 1. GloUf^ma crrtect* ; from the Cballu 



The foramina in the calcareous shell present various appearances. 

 Sometimes they are large and conspicuous (Koialina ytobularii, fig. 13) ; 

 at other* they are so imall that their existence is only to be demon- 

 strated by means of high magnifying powers. Through these fora- 

 mina, long delicate processes of the soft animal, termed pseudopodia, 

 arc protruded. The exact use of these, whether for tactile, pr. h 

 and locomotive purposes, or for the imbibition of nutritive fluid, ia not 

 very clear ; but they very probably fulfil in some degree each of these 

 functions. They may be regarded as analogous to the prolongation* 

 which the Proteus Animalcule (Anwrba) extends in various direction* ; 

 only in the Foraminiftra these organ* are more delicate as well as 

 more uniform in thickness than in the shelless creature referred to. 



Professor Williamson has also demonstrated the existence in several 

 species of a curious system of interspace* and branching tubes, which 

 ramify amongst the calcareous layers forming the walU of the shells. 

 (Horizontal section of Faajatina, fig. 14.) These are especially 

 obvious in the genera Faujanna, Optroilina, and AmphitUgitta. The 

 tubes open at the exterior of the shell, especially at the peripheral 

 margins, either by a few large or by numerous small aperture*. 

 These canals are probably designed to admit water to the interior 

 segments of the animal, with which they communicate through (!> 

 minute foramina. In some coses the pseudopodia are prut: 

 through such of these canals as are situated in the umbilical region ; 

 hut. these appear to be exceptional instances. 



The relations of the Foraminifera to Palicontology render tin-in 

 interesting objects to the geologist Many of the more recent calca- 

 reous strata chiefly owe their origin to the accumulation, through 

 successive ages, of these minute atoms. The White Chalk Hocks are 

 mainly composed of them ; vast ranges of Tertiary Strata present the 

 same characteristic features ; and though the older Limestones have 

 been so altered by pressure and chemical agents that their origin is 

 lea* clear, there ore many indications that they have primarily resem- 

 bled the rocks of more recent age an inference that is n-i 

 probable by the great extent to which sediments now accumulating 

 in the bottom of the sea are charged with these little organisms, and 

 in some cases entirely composed of them. 



The Fossil Foramittifera are chiefly distinguishable from recent ones 

 in the greater prevalence of specimens of comparatively large size. 

 Though one recent species was brought from Borneo by Sir E. Belcher 

 measuring more than 2 inches in diameter, the living forms usually 

 range from the Jth to the -,-,', 1' of an inch. But the Tertiary Strata of 

 the earth abound in examples of the fossil genus Nutnmulite (A T m- 

 mtiiina, fg. ] 5), so called from their resemblance to coins, which vary 

 from the }th of on inch to the size of half-a-crown. These are often 

 so abundant as to form mountain masses, extending through the Alps, 

 Northern Italy, Greece, Syria, Egypt, and Northern India. Thr 

 Mokkadam Mountains in Egypt, where the stone used in building the 

 pyramids was obtained, chiefly consist of these Nummulites, which 

 are known to the natives by the name of Pharoah's Pence. 



Xiimmutilci trntimlarit. 



The structure of the Nummulites has been investigated by Mown. 

 Jolie and Leytneric, and especially by Dr. Carpenter, whilst the <: 

 forms have been studied by M. iJ'Archinc. The genus belongs to the 

 group of the order Hrlirott;/a, in which the outer convolutions com- 

 pletely embrace the earlier-formed one* ; hence it in only by making 

 microscopic sections, or thin slices, that their structure can be fully 

 seen. When such a section is carried horizontally through the 

 of the shell the segment* present a spiral arrangement ; they a* well 

 as the convolutions being remarkable for their small size and conse- 

 quent great number. In other respects they present few or no eswnt i.il 

 differences distinguishing them from more recent forms. A still more 

 curious genus, known by the name of Orbitoida, occurs in America, 

 Switzerland, and India; in the former of which countries it appcu-s 

 largely to represent the Nummulites of the Old World. The labours 

 of I >r. Carpenter have revealed a remarkable structure in this genus, 

 but one that appear* to have some recent representatives. 



Forms of Foraminifera have been found in every formation from 

 the Silurian to the Tertiary. In mot countries the superficial marls 

 and nilts which are associated with prat arc found to be full of the 

 siliceous shields of IHatomacrir, and with these are found the calcareous 

 hells of Foraminifera. They diminish in numbers as we pass below 

 the Chalk ; perhaps also they recede more from the forms of exist- 

 ing minute Poli/thalamarta, but this is a point requiring further 

 investigation. The most abundant forms in the Chalk belong to 

 Kotalia, Spirolina, and Te.rtilaria. In the Oolites and Mountain 

 Limestone the forms generally resemble those of TextHaria. 



