On the Structure and Position of Eozoon Oanadense. 293 



times pass off very obliquely, or even tangentially, so as to 

 run for considerable distances in the chamber-walls ; and their 

 asbestiform casts thus form elongated bundles lying on the 

 surfaces of the segmented layers of serpentine. 



Between the proper walls of the successive tiers of 

 chambers, there usually intervene layers of very variable 

 thickness (Uncoloured Plate, Fig. 1, c, c), composed of 

 homogeneous shell-substance ; these represent the "inter- 

 mediate" or "supplemental" skeleton, which I have described 

 in several of the larger Foraminifera, and which attains a 

 peculiar development in Calcarina.* This is an exogenous 

 deposit on the surface of the proper wall of the chamber, 

 which seems to be formed by the sarcodic layer that originates 

 in the coalescence of the pseudopodia after they have issued 

 from its tubuli, and which is traversed by a more or less 

 minutely distributed " canal-system," occupied during life 

 by prolongations of that sarcodic layer. This canal-system 

 is often brought into view in thin transparent sections of the 

 shell (Uncoloured Plate, Fig. 1, e, e) ; but as the plane of section 

 will seldom coincide with any considerable part of the course of 

 the passages, a much better idea of their distribution is gained 

 from the study of decalcified specimens, which present us 

 with siliceous models of the sarcodic extensions that occupied 

 those passages in the living animal. These extensions cer- 

 tainly originate in some cases in the sarcodic segments oc- 

 cupying the cavities of the chambers ; in other instances I 

 find them to proceed from the stems formed by the con- 

 vergence of the brush-like tufts of pseudopodia ; but more 

 commonly they seem to have sprung (as I have shown to be 

 probably the case in the recent Calcarina) from the sarcodic 

 layer, which is formed by the coalescence of the pseudopodia 

 on the outer surface of the proper wall of the chamber, this 

 layer being represented in the decalcified specimens by a thin 

 plate of serpentine that is often found resting on the extremities 

 of the asbestiform bundles. They differ very remarkably in size 

 and form : being sometimes slender cylindrical rods, which come 

 off from the subjacent layer at regular intervals, and pass straight 

 onwards into the shell- substance without either sub-division or 

 junction ; sometimes presenting themselves as broad flattened 

 plates, which gradually thin out to a sharp edge ; but being com- 

 monly more or less arborescent, and often presenting either 

 beautiful denditric ramifications (lower part of Coloured Plate, 

 andUn coloured Plate, Fig. 3) or a sheaf- like divergence of their 

 component filaments. All these representatives of the sarcodic 

 prolongations that occupied the canal- system are distinguished 



* Memoir on Calcarina, in the Philosophical Transactions, 1860, p. 553 ; 

 and Introduction to the Study of the Foraminifera, p. 220. 



