DESCRIPTIVE CATALOGUE OF RECENT FORAMINIFERA. 255 



The third type of structure is a true shell, composed almost entirely 

 of ciirbouates of lime and magnesia separated by the animal from their 

 solution in the sea water, and fixed in solid form. It is through the 

 agency of the forainiuifera i)rincipally that the limestone, which is con- 

 stantly being dissolved by rains and carried by rivers to the sea, is 

 restored to the solid crust of the earth. Of these ''calcareous" shells 

 there are two kinds, quite distinct in appearance, known as "porcel- 

 lanous" and "hyaline." The former are usually white, opaque, shining 

 with the peculiar luster of porcelain, and "imperforate;" the latter are 

 transparent, glassy, and "perforate," more or less densely, by minute, 

 parallel, unbranclied tubes for the passage of delicate pseudopodia. In 

 both kinds there are usually one or more comparatively large, general 

 ai)ertures. Surface marking, or "ornamentation," is common in both 

 the porcellanous and hyaline shells. In tlie former they take the form 

 of striations or plttings, more or less regular and conspicuous; in the 

 latter, of ridges, tubercles, or s])ines, of clear nontubular shell-substance, 

 varying constantly in number and prominence among individuals of the 

 same species. 



Architecturally the first and most obvious division of these shells is 

 into single-chambered (monothalamous, or unilocular) and many-cham- 

 bered (polytbalamous, or multilocular). While the primitive form of 

 both the single and many chambered shells is evidently globular, yet 

 the possibilities of ultimate conformation, depending chiefiy upon direc- 

 tion of growth, are very great. Thus a monothalamous shell, beginning 

 as an incomplete spherical chamber, may become ovate, fiask-shaped, 

 spindle shaped, star shaped, or tubular, and the tubular form may be 

 straight, curved, coiled, or quite irregular. And these forms pass from 

 one into another by quite insensible degrees. The polythalamous shell 

 is a consequence of the process of reproduction by "gemmation," as 

 the other is of reproduction by "fission." In this case the growing 

 sarcode pushes outside the initial chamber until at a certain stage it 

 builds a new wall around itself, while still maintaining connection with 

 the parent cell. This second segment may give origin to a third, and 

 so on until a colony is established, each offspring occupying an apart- 

 ment added to the parental home. It is easy to see that the style of 

 architecture of these tenements nmy be almost infinitely varied by vary- 

 ing the shape and position of these annexes. Each annex may have 

 any of the forms of the monothalamous shells or any modification of 

 them, and the arrangement may be in straight or curved lines, in con- 

 centric cii'cles or planospiral coils, in single or double series spirally 

 coiled, in two or three alternating series not spiral, or even in an irreg- 

 ular and disorderly mass. 



Usually in the development of the polythalamous shell each succes- 

 sive segment uses the party walls of the preceding segments, so far as 

 they may be available, in the construction of its own annex, but in 

 some of the higher types of the hyaline series it will be found that 



