of the Uiifjn^n to thi; Jiving Zotiiithece. 



38; 



median dark line, donotino- the two axial or directive septa, 

 and on each side of this two other dark lines, which repre- 

 sent two bilateral pairs of septa. In later sections the single 

 median line becomes replaced by two nearly equal lines 

 (fif?- 3)> "hich soon show the enormous difference in size 

 characteristic of the "main" and "counter" axial septa of 

 this species (fig. 4). In the earlier sections the actual 



Fjg. 4. 



Transverse section of the same specimen, taken slill fuithor from the apev 

 than that represented in the former figure. The larger or chief 

 septum now extends much beyond the centre of the nearly solid 

 calice ; the opposite or counter septum is comparatively >mall ; two 

 well-developed additional septa (B) occur within the middle inter- 

 septal space on each side. The superficial ridges and grooves are 

 moderately well marked in the section, and reveal that the median 

 dark line of each septum corresponds with a groove and that two 

 ridges occur between every two adjacent septa. 



boundaries of each septum are not recognizable, but in trans- 

 mitted light the dark median line representing the centres of 

 calcification is very distinct all the way. 



It is also possible to determine the ultimate fate of the two 

 bilateial pairs of primary septa. One pair forms the " alar " 

 or lateral septa of paUtontoIogists, while the two moieties of 

 the remaining pair, recognized by Ludwig and Pourtalfes, but 

 not accepted by Kunth, are disposed one on each side of the 

 axial septum on the convex side of the calice. 



The sections taken at a short distance above the tip indi- 

 cate that the subsequent sej)ta are added within only four of 

 the six primary interseptal chambers or systems, according to 



Ann. tD May. N. Hist. Ser. 7. Vol. ix. 27 



