288 Mr. IT. B. Brady on a new 



tube of equal thickness throughout ; but in reality the exterior 

 is even and smooth. A transverse section of the fossil (PI. XI. 

 fig. 4) shows that this is due to a somewhat remarkable 

 thickening of the shell-wall, especially on its lateral surfaces, 

 most observable near the centre of the disk, and usually to a 

 greater extent on one side than on the other. Sometimes the 

 deposit of shell-substance is proportionally so great that the 

 animal has occupied but a small part of the whole test. 



The shell-wall throughout is traversed by a multitude of 

 very minute tubuli. In the thinner portions (shown in PI. XI. 

 tig. 3) these are apparently the ordinary pseudopodial foramina ; 

 in the thicker (fig. 4), though they run in more or less sinuous 

 lines, they are perhaps only the prolongation of the same. 



Quite distinct from these, there is a series of tubes of much 

 larger dimensions, best seen near the ends of a transverse 

 section, as in fig. 5. I cannot state what purpose they serve ; 

 but the existence of two distinct systems of tubulation is a 

 noteworthy fact. The same thing may be observed in some 

 other Foraminifera, in Orbulina for example ; but as that 

 genus has a thin and uniform shell-wall, the two cases may 

 have nothing in common in respect of structural significance. 



I have alluded to the lamination of the shell. In the true 

 Nummulitethis is a character of importance ; for it arises from 

 the prolongation of the alas of the saddle-shaped chambers to 

 the umbilicus of the test, forming with each turn of the spire 

 a fresh and complete investment of the whole. In the new 

 type (Archcediscus) a tendency to a similar condition exists, 

 but developed to a much less marked extent, and with no 

 approach to uniformity. A section of the test highly 

 magnified, as in fig. 6, shows the successive layers of shell, 

 due to the prolongation towards the umbilicus of the crescenti- 

 form edges of the tube ; but the earlier portions of the tube 

 are nearly circular (transversely), and it is only in the later 

 stages of growth, when it becomes concavo-convex, that it 

 assumes this investing character. 



I have been unable to satisfy myself that there is any es- 

 sential distinction, either in structure or function, between the 

 thin shell-wall and the further deposit which makes up the 

 thicker portion ; in other words, I have not succeeded in 

 determining that there is any distinct primary and secondary 

 skeleton as in the Nummulite. It is nevertheless quite possible 

 at times to trace the thin line of the primary wall, even when 

 no difference in structure is observable between it and the 

 immediately adjacent supplementary layer. In the same way, 

 though I have not been able to identify any part of the structure 



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