1895 THE MICROSCOPE 65 



openings on the margin. Of the two genera, Perichla- 

 mydium is characterized by a thin solid equatorial girdle 

 which Porodiscus lacks. In the former also the cham- 

 bers surrounding the central chamber look more like a 

 mulberry-mass of cells than a series of rings, and the 

 form presents the general appearance of fig. 19. (In 

 this figure the pores over the central and surrounding 

 chambers are not shown). In the next section there are 

 still no chambered arms or radial spines, and the main 

 characteristic in the genus from Barbadoes is a single 

 opening on the margin which is surrounded by spines. 

 The nature of the openings is shown in fig. 18 which, 

 however, represents a genus not found in Barbadoes 

 which has three of these openings and no spines. Fig. 

 17 gives some idea of the outline of the Barbadoes form 

 and of the opening and spines. In the next section 

 there are no arms and no openings but radial spines, 

 the number of which, together with the presence or 

 absence of a girdle serves to distinguish the genera. 

 And in the last section there are chambered arms, the 

 number and disposition of which, together with the 

 presence or absence and the disposition of a patagium 

 or membrane, form the distinctive marks of the ge- 

 nera. 



Finally in the Spongodiscida the sieve-plates disap- 

 pear, and the shell is more or less spongy in 

 character. The simple spherical central chamber some- 

 times has concentric rings around it but the surface 

 of the disk shows an irregular spongy framework. 



This finishes the Spumellaria. Next we come to the 

 Nassellaria. 



To be continued. 



