GENUS POLYSTOMELLA : HISTOET. 537 



period in the growth of the shell is there any passage through these " fossettes " to the 

 cavity of the chamber, the only communication possessed by any chamber either with 

 contiguous chambers or (in the case of the outermost chamber alone) with the exterior, 

 being afforded by a variable number of minute orifices (corresponding with the septal 

 pores of Peneroplis} which are to be found near the inner margin of each septal plane, 

 close to its junction with the preceding convolution. Corresponding to the elevated 

 ridges of the crenulations, we find a series of grooves on the internal surface, which 

 shallow towards the anterior or concave margin of each segment, and deepen towards 

 the posterior or convex margin ; and for a short distance from the posterior septum each 

 groove is converted into a tube by a narrow lamella given off internally from the septum. 

 These tubes, however, establish no communication between the contiguous chambers; 

 for they are culs de sac, closed-in by the lamella of the septum which formed the 

 boundary of the previously-formed chamber. In the living state they are occupied (as 

 can be shown by examination of the decalcified body) by a set of processes of sarcode, 

 which extend backwards for a short distance from both the outer or lateral margins 

 of each segment of the sarcode-body, and then terminate abruptly. From the neigh- 

 bourhood of the inner arch of each segment, on the other hand, there proceeds a series 

 of threads of sarcode much slenderer than the " retral processes " just described, which 

 unite each segment to the two contiguous segments before and behind, passing through 

 the row of pores already mentioned as visible along the inner margin of the septum 

 The shell is described by Professor WILLIAMSOX as " crowded with myriads of minute 

 foramina," and as also covered over with small pointed tubercles, which, from the 

 rounded forms of their bases, and their great transparency, may be easily mistaken for 

 apertures in the shell, especially in the " fossettes," where these tubercles are often very 

 large. He further pointed out that the umbilical region is occupied by a solid mass of 

 shelly substance, into which the decalcified animal does not appear to extend, and the 

 surface of which is often marked with small depressed pits, the orifices of vertical 

 internal passages, through which pseudopodia are probably protruded. 



176. It is obvious from the foregoing account that \iPolystomella crispa is to be taken 

 as the type of the genus, the generic definition given by M. D'OKBIGNY is based on an 

 entire misapprehension of its true structure ; the only considerable departure from the 

 general type of Helicostegue structure being the substitution of a series of isolated pores 

 for the ordinary single orifice of communication between the successive chambers (a 

 difference which in the case of Peneroplis and Dendritina we have seen to have not even 

 a specific value), and the supposed lateral orifices having no real existence. 



177. Subsequently to Professor WILLIAMSON'S memoir, an elaborate account of the 

 characters of the genus Polystomella, and especially of a species designated P. strigilata 

 (which seems to me only one of the multiform varieties of P. crispa), has been given by 

 Professor MAX SCHULTZE, in the excellent treatise ' Uber den Organismus der Poly- 

 thalamien,' to which I have already referred. He had the advantage of being able to 

 study this species in the living state ; and he has thus been enabled to give a beautiful 



MDCCCLX. 4 B 



