FORAMINIFERA. 885 



families where pores are present, ' perforate/ designations sometimes em- 

 ployed as classificatory. There is an organic basis, more or less plentiful, 

 visible after careful decalcification, and a distinct chitinoid membrane of 

 varying thickness lines the cavity of the test, its passages and pores \ The 

 principal mineral constituent is Calcium carbonate, as Calcite in the perforate 

 test, but possibly in the form of Arragonite in the imperforate. Among 

 Miliolidae, Biloculina has been found to contain as much as 7-10 per cent, 

 of Silica, and Orbitolites about 10 per cent, of Magnesium carbonate with 

 a trace of Silica. In the perforate series it has been observed that the 

 tests of pelagic Globigerinae are entirely soluble in acid ; and that those 

 of two Nummulids, Amphistegina Lessonii and Operculina complanata^ 

 contain 5 per cent, of Magnesium carbonate, with traces of phosphates, 

 Silica, Ferric and Aluminic oxides. Grains of sand are in some instances 

 attached to the outside of the test, e. g. frequently in the Miliolid Nube- 

 cularia ; and in the larger Textularinae the quantity of sand thus added 

 may be very great, the perforate calcareous basis sometimes disappearing. 

 The perforate Polytrema and Carpenteria, especially the latter, use sponge- 

 spicules as a foundation for the calcareous crust. In species which are able 

 to live in brackish water the amount of calcareous matter diminishes until 

 in one instance, Miliolina ( = Quinqneloculina) fusca, the test becomes 

 chitinoid with minute impacted sand-grains ; so too in Entzia from a salt- 

 pool at Deva in Transylvania. The tests of Miliolina from abyssal depths 

 (3950 fathoms) in the North Pacific, where the bottom is a Radiolarian 

 ooze, are reduced to delicate homogeneous siliceous films. 



The calcareous test affords many varieties of shape. It may consist 

 of a single or of many chambers ; in other words, it is mono- or poly- 

 thalamous, a distinction which has been used for classificatory purposes, 

 but has no significance so far as the intimate structure of the organism 

 is concerned (infra, p. 891). It is nearly always free, seldom attached, 

 and its growth as a rule follows a definite plan. Sometimes however 

 it is really irregular, though outwardly symmetrical, as in the lenticular 

 Nummuline Archaediscus, which consists of a tube coiled on itself in 



1 As a rule no signs of structure are to be discerned in the test save the lamination of many 

 perforate forms. In a few instances, the perforate Amphistegina, Opemilina, Heterostegina, 

 Cycloclypeus, it is composed of vertical hexagonal prisms traversed each by a pore ; and its outer or 

 secondarily deposited layers in Orbulina, Globigerina, &c., are said to be made up of vertical 

 wedge-shaped masses. The siliceous skeletons of diatoms, sand-grains, &c., have been detected in 

 the tests of the perforate Amphistegina and Orbulina during decalcification (Folin, C. R. 102, 

 1886, pp. 1575-6). The specific gravity of the imperforate test is higher than that of the perforate ; 

 the former varies from 2-7 to 2-722, the latter from 2-62.6 to 2-674 (Sollas, Sci. Proc. Royal Dublin 

 Soc. iv. 1885, p. 390). Little is known as to the way in which new chambers are added. The proto- 

 plasm grows in amount and protrudes from the last chamber, and the walls of the new chamber are 

 at first very thin ; the pores are evident at an early period ; see Max Schultze, Organismus, &c., pp. 

 29-30. Growth in size mu-t take place by resorption within and addition without. According to 

 Moebius (Beitrage, &c., p. 83), the pores in the arborescent stem of Carpenteria raphidodendron are 

 formed soon after the calcareous matter is first laid down. 



