16 ■ FORAMINIFERA 



corbis, etc., where there is a depressed umbilical area or aper- 

 tural face, specialized costae running in toward the aperture are 

 developed, often becoming highly ornate. 



In the old age of individuals or in the later development of 

 species the ornamentation is often lost as in other animals, and 

 the test that was highly ornamented in the early stages becomes 

 gradually less so and in the adult entirely smooth (PL 35, fig. 

 17). Such a disappearance of characters does not take place 

 equally in all parts of the test. In coiled forms of the Lagenidae 

 for example, the ornamentation on the general chamber surface 

 becomes smooth first on the proximal side and the ornamenta- 

 tion is held longest on the peripheral part. In the same speci- 

 men the ornamentation of the suture lines disappears first in the 

 peripheral part of the suture, and is retained longest on the 

 proximal end, an exact reverse of the conditions on the chamber 

 wall between the sutures. 



In many species the ornamentation is closely identified with 

 the individual chambers and sutures or patterns are broken at 

 the edges of the chambers (PI. 36, fig. 27). In others, the orna- 

 mentation belongs to the test as a whole and is continued un- 

 broken from chamber to chamber (PI. 35, fig. 11). The large 

 spines may be added to as each chamber is built so that they 

 have a laminated appearance in section. 



Aperture. The aperture of the test is one of its most import- 

 ant parts from the standpoint of relationships and descriptive 

 work. It is the opening through which the main body of pro- 

 toplasm has its chief egress to the exterior. The aperture in 

 the adult of a given species is rather constant when its devel- 

 opment is known. The fact that the aperture changes as the 

 test develops has been held to be a feature which would deprive 

 the aperture of much consideration as a feature of systematic 

 descriptive worth. The fact that its changes are usually in a 

 logical sequence and that these very changes are capable of in- 

 terpreting relationships has often been overlooked. The young 

 of a complex form will have in its early stages the simple aper- 

 ture that is characteristic of the genus related to that stage in 

 its development. Thus in Pyrgo, the early quinqueloculine stage 

 will have the simple tooth characteristic of Quinqueloculina, in 

 its next or triloculine stage, the bifid tooth characteristic of 



