158 Proceedings of the Itoi/al Irish Academy. 



the completed plan. Such progressive multiformity is admirably shown in 

 the genera Peneroplis, Orbiculina, Orbitolites in the family Peneroplidinae, 

 and in their perforate isomorphs Opereulina, Heterostegina, Cycloclypeus in 

 the family Xummulitinae. 



Reti-ograde multiformity, assuming the transition from a complex to a 

 simple plan of growth, is well represented in many genera of the family Textn- 

 laridae — for example, Spiroplccta, Clavulina, and Gaudryina; but perhaps 

 nowhere better than in the genus Orbiculina, which, in addition to the multi- 

 formity alluded to in the last paragraph, exhibits a form of variation which has 

 been briefly alluded to by Carpenter" and Lister," and which is probably 

 unique in its kind. The generic distinction between Peneroplis and Orbiculina 

 lies in the subdivision of the chambers in the latter genus into chaniberlets 

 by the growth of internal septa. Apart from this distinction, there is little 

 difference observable between small individuals of Orbiculina and specimens 

 of Peneroplis of the arklimis group. Carpenter first noted that the septa 

 dividing the chambers into chaniberlets are sometimes wanting, "not merely 

 in fully developed peneropliform varieties, but even in good-sized adunciform 

 specimens," which, as he remarks, " is a fact of not a little significance." 

 He proceeds to state his deductions that " in such cases no absolute line of 

 demarcation can be laid down between Peneroplis and Orbiculina ; for 

 although there may he practically little or no difficulty in referring any 

 given specimeu to one or tlie other type by the aggregate of the characters 

 it presents, yet no one of these characters taken by itself is sufficiently 

 constant to serve as the basis for a precise definition." 



With this remark we entirely agree. No xhizopodist familiar with the 

 appearance of both genera would have any hesitation in referring these 

 abnormal forms to their correct generic position in Orbiculina. And yet, 

 structurally, they are not Orbiculina, but Peneroplis ; and in them the 

 Chinese wall separating the two genera, normally quite diflerent in outward 

 appearance and internal etnicture, is broken down. 



Lister adds very little to our knowledge of this particularly interesting 

 form of variation. He states that in some "stunted forms" of Orbiculina, 

 "though by no means in all," the "subdivision into chaniberlets may be 

 incomplete or wholly absent. Sometimes the subdivisions die out in the 

 terminal chambws after Ijecoming established in their predecessors; in 

 others it is absent throughout the test. 1 am inclined to regard these latter 

 forms as examples of Orbiculina which have lost their secondary septa by 



'* W. B. Carpenter, W. K. Parker, and T. Rupert Jones : ' ' Introduction to the Study 

 of the Forarainifcra " (Ray Society). London, 1862, p. 98. 

 " Loc. cit., note 5, p. 100. 



