an 



FORAMINIFERA. 



FORAMINIFERA. 



menta U wound round the primary central one, u the thread is around 

 a ball of worsted. (Q*i*q*elociUuia, Trilocuiinn.'i 



In the great majority of the species the interior of each chamber is 

 simple and undivided, but there are dome forma, especially amongst 

 the Ilrlicotteyn, in which the newer and more external chambers are 

 subdivided either by transverse pillars or by complete partitions 

 perforated by one or more aperture*, through which prolongations of 

 the gelatinous substance unite the various segments of the soft animal. 

 (Orbifulino, f<j. 11.) Ordinarily but one such chain of communica- 

 tions exist (animal of Rtuolino, fa. 12); but in the cases just referred 

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

 the septa become completely cribriform. The distribution of these 

 apertures affects the gemmation or mode of growth, since it is 

 through them that the new segments arc 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 Orbicnlina, this growth is sometimes so 

 remarkable that the new segments soon form concentric circles, 

 embracing all those previously formed. (Orljiculina complanala, Jig 1 1 .) 

 In such examples the connecting apertures are distributed round the 

 entire periphery, and gemination 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 is 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 

 ha; the power of diffusing itself over the shell, aud depositing upon 

 its surface additional layers of calcareous matter. 



lla 



The foramina in the calcareous shell present various appearance*. 

 Sometimes they are Urge and conspicuous (Kotalina globularii, fig. 1 3) ; 

 at others they are so small that their existence is only to be demon- 

 strated by means of high magnifying power*. Through these fora- 

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

 arc protruded. The exact use of these, whether for tactile, prehrn-i :. 

 aud locomotive purposes, or for the imbibition of nutritive fluid, 

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

 functions. They may be regarded as analogous to the prolongations 

 which the Proteus Animalcule (Anurba) extends in various directions; 

 only in the Foraminifera these organs 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 interspaces and branching tubes, which 

 i ramify amongst the calcareous layers forming the walls of the shells. 

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

 obvious in the genera Faujatina, Operculina, and AmjJiutfyina. The 

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

 margins, either by a few large or by numerous email apertures. 

 These canals are probably designed to admit water to the interior 

 segments of the animal, with which they communicate through the 

 minute foramina. In some cases the pseudopodia are protruded 

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

 but these appear to be exceptional instances. 



The relations of the Foraminifera to Paltcontology render them 

 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 

 less clear, there are many indications that they have primarily resem- 

 bled the rocks of more recent age an inference that is rendered 

 probable by the preat 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 Foraminifera are chiefly distinguishable from recent ones 

 in the greater prevalence of specimens of comparatively lni>; 

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

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

 range from the Jth to the yAsth of an inch. But the Tertiary Strata of 

 the earth abound in examples of the fossil genus Nummulite (.Yum- 

 mnlina,jig. 15), so called from their resemblance to coins, which vary 

 from the Jth of an 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. The 

 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. 



IS 1G 



8. Cauiduliiia. 9. Tezlilarla. 10. SflrolonliM. 11. Orbiculina com. 

 flaiuila. lla. Part of two chamber* of in OrUeuIina. 12. Dccalclnrd 

 animal of a Ronaltna. 1 3. Rontlina globular!*, rif wrti a< a transparent object 

 fhoin(c the foramina. 14. Horizontal ncction of Ki(f. 6, allowing the internal 

 TMm of tube*. 19. Fowll Xummu/ma. 1(1. Vertical arrtion of Fig. 1:,. 

 It. rrrmftifilitta tri&irinala ; from the Chalk. 18. Koialina Lortirtaria, 

 from tb Chalk. U. GloUftrina creUuet ; from the Chalk, 



yummulitct laitictilaiit. 



The structure of the Nummulites has been investigated by Messrs. 

 Jolie and Leymerie, and especially by Dr. Carpenter, whilst the specific 

 forms have been studied by M. IVArchiae. The genus belongs to the 

 group of the order l/rlinidiya, in which the outer convolutions com- 

 pletely embrace the earlier-formed ones ; hence it 1 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 segments present a spiral arrangement ; they as well 

 as the convolutions being remarkable for their smnll si/,c and conse- 

 quent great number. In other respects they present few or no tv 

 differences distinguishing them from more recent forms. A still more 

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

 Switzerland, and India; in the former of which countries it appears 

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

 <>f Hr. Carpenter have revealed a remarkable structure in this genus, 

 but one that appears to have some recent representatives. 



Forms of Foraminifera have been found in every formation from 

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

 and silts which are associated with peat are found to be full of the 

 siliceous shields >t . mid with these are found the calcareous 



shells of Foraminifera. They dimini.-li in numbers as we pass below 

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

 ing minute Paljmalamaam, but this is a point requiring further 

 investigation. The most abundant forms in the Chalk belong to 

 Rotalia, Kpirolina, and Te-rtilaria. In the Oolites and Mountain 

 Limestone the forms generally resemble those of TextUaria. 



