54 THE FORAMINIFERA 



In these the tests are rigid structures, and communicate with 

 the exterior either by one or more large apertures exclusively, as in 

 the majority of the Miliolidea, or by a multitude of small pores in 

 their walls in addition to the large apertures. 1 In some genera 

 of the Miliolidea the shells have a polished white appearance re- 

 sembling porcelain when seen by reflected light, and a yellowish 

 brown horn colour when seen by transmitted light ; in the per- 

 forate forms the tests are more transparent, and are in many cases 

 as clear as glass. On this account the forms with perforate cal- 

 careous tests are often known as the vitreous or hyaline Foraminifera. 



In the perforate forms the pores passing out from the chambers 

 have, on the whole, a direction perpendicular to the walls, and 

 transmit pseudopodia. In some cases the pores are large and com- 

 paratively few, in others they are fine and very numerous. 



The Growth of the Test. In forms such as Myxotheca and Gromia 

 the growth of the protoplasmic body is accommodated by the simple 

 expansion of the soft and membranous test. Among the arenaceous 

 forms, the feebly cemented, star-shaped tests of Astrorhiza (Fig. 17, 

 a) increase in size in part by the extension of the test along the 

 protoplasmic trunks forming the rays of the star, but in part, 

 doubtless, by the expansion of the central body. In the case of 

 Saccammina (Fig. 1 7, b), however, and other forms with rigid single- 



2722. The specific gravity of calcite is 272, and that of aragonite 2'97. He con- 

 eluded that the calcareous constituent of the perforate tests is calcite, and that of the 

 imperforate tests either aragonite or calcite together with some other and heavier 

 substance. Cornish and Kendall (12) had previously indicated the conclusion, though 

 without positively stating it, that the porcellanous Foraminifera were composed of 

 aragonite on the grounds of their opacity, and their appearance in or absence from 

 beds coincidently with Lamellibranch and other fossils which are composed of ara- 

 gonite. Chapman (11, p. 39) also has recently stated that the tests of the Miliolidea 

 are of aragonite, or rather (following Miss Kelly, Mineralogical Mag. vol. xii. (1900), 

 p. 363) conch ite. 



I am inclined to doubt this conclusion. It appears that the presence of the mag- 

 nesium carbonate (specific gravity 3*056), which Brady's analysis shows is a larger 

 constituent of the imperforate tests than of the perforate, may cause the higher 

 specific gravity found in the former by Sollas. Moreover, Meigen (22) has recently 

 described a chemical colour test by whicli calcite may be distinguished from ara- 

 gonite, or from the constituent which Miss Kelly has named conchite. Tried by 

 this test, I find that MUiolina and Orbicidina and Orbitdites do not give the colour 

 reaction characteristic of aragonite, but agree with structures composed of calcite. 

 I am indebted to Mr. A. Hutchinson for calling my attention to Meigen's paper. 



1 In the systems of classification prepared by Reuss (1861) and by Carpenter 

 and his colleagues (1862) the Foraminifera were divided into the two groups, 

 the Imperforata and the Perforata. In the latter classification the group Imperfor- 

 ata includes the Groniida, Lituolida, and Miliolida; the Perforata the Lagenida, 

 Qlobigerinida, and Nummulitida (i.e. families 5-10 of Brady's classification, which is 

 followed iu this article). The progress of deep-sea investigation since this date has 

 revealed the existence of the Astrorhizidea, some of which have perforate and others 

 imperforate tests. Moreover, even in the Miliolidea the first formed chambers may 

 be perforate in the genus Peneroplis (Rhumbler), and in Orbicidina and Orbitolites, 

 aa shown in this article. It thus becomes evident that the presence or absence of 

 perforations in the shell, though perfectly characteristic of many of the orders, cannot 

 be taken as the basis for the subdivision of the whole group. 



