355 



with numerous small irregular apertures leading directly into the loculi or 

 empty spaces between the septa. In the third species, A. Atlanticus, it 

 (the outer wall) appears to have a compact smooth surface, with only a 

 few perforations. The inner wall is very thin, with numerous pores lead- 

 ing from the loculi into the great central cavity. The septa consist of 

 thin flat plates, arranged longitudinally exactly as in the genus Za- 

 phrentis. They extend from the outer to the inner wall, and are 

 perforated with numerous small circular pores, so that the interseptal 

 loculi all communicate with each other as well as with the central 

 cavity and the exterior. The loculi are subdivided by very thin dissepi- 

 ments resembling those of a Ci/athcpJiyllum, but they are irregularly 

 distributed, being in some parts entirely absent, and in other places so 

 numerous that they completely fill the loculi with small cells constituting 

 the "poriferous or cellular tissue" mentioned in the original description 

 of the genus (ante, p. 3). The central cavity extends nearly the whole 

 length, and constitutes a large proportion of the bulk of the fossil. Below 

 it there is a portion of the smaller extremity or base, which is composed 

 only of the outer wall, the septa and the dissepiments. The section 

 across this part shows that the new septa, which are introduced from time to 

 time, as the diameter increases, do not at first extend to the centre, and 

 it would appear from this that they were developed on the inner surface 

 of the outer wall, and gradually widened as in the genus Zaphrentis. 



Fig. 343. Fig. 344. 



Fig. 343. Restoration of the lower part of Archeocyathus Minganensis ; a, the pores of 



the inner wall. 



344. Spicnla found imbedded in the walls of the same species or associated in 

 the same rock-specimens. Enlarged 50 diameters. 



