, 113 



In the parietes of this species we again find a lamellar structure, 

 similar to that described as occurring in Amphistegina gibbosa ; only 

 owing to the circumstance that the convolutions do not embrace and 

 enclose one another so completely in the Nonionina as in the Amphis- 

 tegina, the former object does not show the great increase in the 

 thickness of each umbilical region, which constitutes so peculiar a 

 feature in the latter one : still, a large number of the lamella? can 

 be traced far beyond the permanent limits of the segment, of which 

 they constitute the primary coverings, and are even seen to extend 

 completely across the diameter of the shell. As in Amphistegina, 

 they at least reach the centre of the organism, where they disappear 

 amidst those which have come, in a similar manner, from the other 

 side. This is a more remarkable circumstance even than that 

 recorded in the case of Amphistegina. In the latter genus, as 

 in Polystomella and many others, the newest convolutions completely 

 embrace and enclose each other. In this Manilla shell, on the other 

 hand, every convolution, from the centre to the circumference, is dis- 

 tinctly seen ; and the outline of every septum, as already mentioned, 

 is conspicuously visible. 



The greater part of the shell is perforated with the usual 

 pseudopodian tubes ; but where the section has traversed any 

 of the septa at right angles, or nearly so, we find that in the 

 portions of the shell above and below each septum so divided, the 

 tubes are wanting, producing the peculiar appearance seen at fig. 

 4, d. The absence of the foramina from these points has caused the 

 laminated texture of the shell to be more obvious, and at the same 

 time has rendered these parts of the organism more translucent 

 than the rest ; a condition which accords with that noticed to 

 exist in Amphistegina, where the absence of the tubes produces 

 a similar effect, modified only, as to the arrangement of the trans- 

 lucent portions, by the differences between the contours of the 

 two objects. The real nature of this structure in the Nonionina is 

 still better shown by fig. 5, which represents a small part of a verti- 

 cal section made in a line parallel to that of fig. 4, only midway be- 

 tween the centre and the circumference of the object, so as to 

 traverse at right angles the septa of the more external convolutions. 

 Here we see that each septum is, as usual, double (5, b), and the 

 pseudopodian tubes arising from the cells on each side of the sep- 

 tum at first converge as they ascend (5, a), and afterwards diverge as 

 they approach the outer surface, thus leaving the translucent space 



TRANS. MIC. SOC. VOL. III. Q 



