of the Shell of Operculina Arabica. 169 



Intercameral communications numerous in the septa of the 

 last-formed whorl, the largest long, narrow and crescentic, 

 arching over the margin of the preceding whorl. 



Dimensions. — l-6th of an inch in horizontal, and l-96th in 

 vertical diameter ; widest part of last whorl l-24th of an inch 

 (PL IV. fig. 1). 



Observations. — This description is chiefly taken from one of 

 the largest and most regularly formed shells I possess. They 

 are by no means always plane, but, on the contrary, frequently 

 wavy, like Nummulites ; and the chambers sometimes increase 

 in size more rapidly than at others, causing the shell to assume 

 a more or less elongated or oval form ; the chambers are also 

 sometimes broader, sometimes narrower ; and occasionally a sep- 

 tum only extends part of the way out towards the margin of the 

 whorl, when it suddenly bends backwards to meet the preceding 

 one, or it may stop short altogether, and then the chamber be- 

 hind and before it coalesce at their outer parts. Irregularities 

 of this kind in the formation of the chambers of Forarainifera 

 are not at all uncommon, and apparently so usual in Num- 

 mulites lavigata, that they would seem to constitute a character. 

 The imperfectly developed chamber extending from the margin 

 of the foregoing whorl outwards instead of in the opposite di- 

 rection, seems to point out the course in which the chambers are 

 formed ; and if each chamber is to be regarded only as the full 

 development of a single animal, the imperfect one must be consi- 

 dered as an abortion, and those which have coalesced as a mon- 

 strosity. Most frequently there is here and there a large open- 

 ing in the shell, over one or more of the chambers, which leads 

 into the latter ; they are more or less round, larger or smaller, 

 and the smoothness of their margin would seem to indicate that 

 they had been formed by the animal itself, if not by some other 

 animal. 



Microscopic Examination. — The chambers of the shell, after 

 the green cuticular substance has been removed, are found to be 

 covered externally with large and small papillae ; the former 

 1-21 50th, the latter l-8600th of an inch in diameter (fig. 2). 

 The former also ai'e about twice their own breadth apart, and 

 the latter occupy the intervals between them ; both are con- 

 fined to the arese over the chambers ; they do not appear over 

 the septa nor on the margin of the shell. The large papillaj 

 appear to be imperforate, while the small ones appear to present 

 each a puncture in the centre. The septal spaces, as well as the 

 central cell, are semitransparent, and the former have a single, 

 beaded line of semitransparent papillae along their course. 



The internal surface of the chambers merely presents the small 



