PROTOZOA. 33 



The Miliolidae are represented by the mounted speci- 

 mens (No. 21). The term Miliola may be used very 

 properly in a generic sense to comprehend a great variety 

 of closely associated forms having the same general type 

 of structure (Brady). It is reasonable to suppose that a 

 single form, Uniloculina or Loculina, exists, or has existed 

 in the past, although no such form has been described. 

 The Biloculina (No. 21) has two chambers visible exter- 

 nally, and each successive segment encloses the younger 

 ones on the same $ide. 



The group of Foraminifera is a very remarkable one 

 for studying gradational forms. Here, to the inexpressi- 

 ble delight of the student, all artificial systems break down. 

 "It is only/ 7 says Brady, "as we learn to recognize the 

 fact that among the Rhizopoda the so called ' species ' rep- 

 resent no more than terms of a series of which very fre- 

 quently every intermediate link can be supplied that we 

 arrive at any just idea of their relationship." l Among the 

 more specialized groups of animals many of the interme- 

 diate forms are unfortunately wanting, but these doubtless 

 either exist at the present time or have existed in the past, 

 and if the lesson taught by the Foraminifera could be 

 impressed upon the student at the beginning of his studies, 

 he would be less inclined to draw sharp lines of demarca- 

 tion, since these are arbitrary and unauthorized by nature. 



No. 22 and PI. 23, figs. 1-7, represent Peneroplis, of 

 which Brady says there is no genus of Foraminifera 

 embracing so great a variety of external form in which the 

 morphological sequence is at once so simple and so com- 

 plete. 2 



PI. 23, fig. i, is a young specimen of Peneroplis show- 

 ing the spiral mode of growth. Fig. 2 is an adult of the 



1 Challenger Report on the Foraminifera, IX, 1884, p. 49. 



2 For other figures showing the variety in external form of this 

 genus, see Brady; Challenger Report on the Foraminifera, IX, 

 1884, PI. XIII; also Carpenter, Introd. to Study of Foraminifera, 

 PI. VII. 



