GENUS POLTSTOMELLA HISTOEY. 
537 
period in the growth of the shell is there any passage through these “ fossettes ” to the 
ca\ity 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 cuts de sac, closed-in by the lamella of the septum which formed the 
boundary of the preriously-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 Williajvison 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 if Polystomella crispa is to be taken 
as the type of the genus, the generic definition given by M. d’Orbigny 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 Pener(yplis and Pendritina we have seen to have not even 
a specific value), and the supposed lateral orifices having no real existence. 
177. Subsequently to Professor Williajmson’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. cris'pa), has been given by 
Professor MiVs; 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 
