FORAMINIFERA. 887 



aspect, successive coils inclosing their predecessors completely. The im- 

 perforate Keramosphaera is spherical, and consists of chambers arranged in 

 concentric layers ; there is no corresponding form known in the perforate 

 series. An altogether irregular arrangement of chambers obtains in the 

 perforate Tinoporinae, and irregular layers of chambers are found on each 

 side of the disc in Orbitoides. 



Dimorphism exists in some cases, but the term has been applied in 

 two different senses. In one it denotes that a given test follows in its 

 individual growth two different types of structure, in the other that it 

 is found under two different forms. The first mode is exemplified in the 

 Miliolid Hauerininae, some Texttdaridae , and Lagenidae, e.g. Hauerina^ 

 with its first chambers arranged as in Miliolina, their successors in a 

 planospiral with more than two in each turn of the coil. Trimorphism 

 in the same sense is very rare. The second mode has been observed in 

 Numtmdites and its congener Assilina. The tests, e. g. of Nummulites, 

 occur in pairs, one constantly smaller than the other and possessed of a 

 large primordial chamber, the other of larger size but with a small 

 primordial chamber or none at all recognisable. The Miliolininae combine 

 both modes. One set of forms is small, the other large ; the former have 

 a large, the latter a small primordial chamber, with, moreover, the first 

 chambers differently grouped to their successors which are typical. The 

 initial chambers, moreover, in one and the same species may be disposed 

 in different ways, leading to a polymorphism. The significance of these 

 facts is not known. 



As to structure, the following points may be noticed. The terminal 

 aperture of the test is rarely absent as in many specimens of Orbulina ; 

 it may be simple, its shape sometimes depending on that of the test, 

 sometimes independent of it ; or it may be radiate, owing to the projection 

 across it of calcareous bars ; finally, it may be replaced by pores. In 

 Lagena great variety prevails ; it may be situated at the extremity of a 

 tube (ectosolenian), or the tube is prolonged inwards into the test (ento- 

 solenian) ; whilst in other examples the two modes may be combined (ecto- 

 ento-solenian), and more rarely a second aperture is present 1 . The 

 spherical or pyriform chambers ^iRamulina have several tubular orifices, and 

 in the arborescent Carpenteria raphidodendron and Polytrema the branches 

 terminate in simple apertures. In a few chambered tests each chamber 

 has its own separate aperture, e. g. in some species of Globigerina ; and in 

 Cymbalopora not only is this the case, but accessory apertures may be 

 present as well. The chambers in polythalamous tests may be separated 

 from one another by slight constrictions, by imperfect septa, or by perfect 

 but porous septa. They may in some instances be subdivided into 

 chamberlets by vertical partitions, imperfect in the imperforate Orbiculina, 



1 Gruber has described an entosolenian Gromia Lagenoides ; Nova Acta, xlvi. p. 495. 



