FORAMINIFERA OF NORTH PACIFIC OCEAN. 5 



group, as will bo shown in certain of the families to be taken up later, 

 when the details will be discussed at length. The term " dimorphism " 

 would hardly cover the case in some genera, where eight or more dis- 

 tinct stages may be made out, each with its characteristic form of 

 chamber, yet all appearing successively in a single test. 



The number of chambers in the complex tests varies from a few to 

 a great many. Where the size of the test becomes considerable and the 

 chambers correspondingly large, the chamber is often divided up in 

 various ways into chamberlets, as in Orbitolites. In such cases the 

 adjacent chamberlets are usually in free communication with one 

 another. The walls of the chamberlets give additional strength in 

 many forms in which they are developed. Another characteristic 

 modification in some genera is the development of labyrinthic struc- 

 tures in the interior of the chambers. Such structures are seen in 

 Oyclammina, Haplostiche, Fabularia, etc. In general, it seems to be a 

 mark of the culmination of certain lines in development, and inany 

 of the genera which developed such labyrinthic structures are now 

 extinct. From the appearance of a series of such tests of one species 

 at different stages in development, it would seem as though this 

 labyrinthic condition was developed as a secondary growth in the 

 chamber. One of its uses may be to give added strength to the test, 

 but this does not always seem to be the case, for it may occur in tests 

 which are characterized by thick walls. 



APERTURES. 



The aperture in a given species seems to be rather constant when 

 the development is understood. Much has been written upon this 

 subject; apertural characters have been used by some authors as a 

 basis for systematic work and discarded by others as very variable. 

 In a few specimens it may seem at first sight as though the apertural 

 characters were very variable, but with a large series showing different 

 stages in development another phase of the matter is presented. In 

 certain cases there is a very decided change in the condition of the 

 aperture, but these changes appear at definite stages in the life history, 

 and all may be seen by cutting back a single full-grown individual. 

 In general, it has seemed from the present study that apertural char- 

 acters, when studied in large series, are a rather dependable set for 

 systematic work, and this will be shown to be true in the Miliolidaa 

 and Lagemdae especially. 



In many species teeth of various sorts are developed in the aperture, 

 and these teeth are subject to various modifications. It can be dem- 

 onstrated that these modifications occur in a definite sequence, and 

 that this sequence is important from a plrylogenetic point of view. 



In a considerable number of genera a definite tubular neck is devel- 

 oped, with the aperture at its end. This neck is seen in many genera 



