120 



added, until the organism attains to the large size already spoken 

 of; those of a more recent growth differing only from the circles first 

 formed, as stated above, in the existence of lateral channels of com- 

 munication, corresponding with those seen in O. complanata. 



When we compare the development of these Orbiculince, as now 

 described, with that of other well-known Foraminifera, we see that 

 although there are peculiarities attending the details of the former ex- 

 amples, yet they strictly conform to one general type. The existence 

 to a central cell, more or less distinct from those by which it is suc- 

 ceded, has been previously shown, by both M. D'Orbigny and myself, 

 to characterize nearly all the Foraminifera. On the other hand, I am 

 unacquainted with any of the Bryozoa whose polypidom exhibits 

 such a structure. However numerous may be the individual cells thus 

 aggregated into one common polypidom, they are all exact repetitions 

 one of another, excepting that amongst some of the Eschara the cal- 

 careous cells of the older members of the group may be, as Milne- 

 Edwards has shown, somewhat thicker than those of a younger growth ; 

 but there was no primary difference between the first cell enclosing 

 the original germinal polype, and those which subsequently grew 

 up around it. Thus we may legitimately conclude that whilst the 

 structures represented in figs. 10 and 14 find no analogues amongst 

 the polypidoms of the cilio-brachiate polypes, they do exhibit a close 

 conformity with the typical contour which prevails amongst the true 

 Foraminifera. That this is correct reasoning is, however, placed 

 beyond a doubt by a comparison of the structures just described with 

 that existing in Orbiculina adunca. This elegant Foraminifer has 

 long been one of the best known of recent species. Very fair figures 

 of it, both in its young and half- developed states, are given by Fich- 

 tel and Moll ; the former bearing the name of Nautilus angulatus* 

 and the latter that of N. adnncus.\ Excellent figures of some of its 

 protean forms are also given in M. D'Orbigny's beautiful work on 

 the Foraminifera of Cuba. In its young state it is one of the most 

 abundant organisms in the Cuban sand. Fig. 15, which represents 

 one of these immature forms, shows that it is at first a Foraminifer 

 of the ordinary spiral type. Gradually, however, the posterior 

 angles of the new segments, instead of continuing to be prolonged 

 over the dorsum of the shell, as at fig. 15, a, begin to terminate 

 more abruptly, as at fig. 15, 6. This change becomes increasingly 



* ' Testacea microscopica,' &c, tab. 22. His N. orbiculensis is probably a variety 

 of the same. 



f Idem, tab. 23. 



