ioo THE FORAMINIFERA 



correlated with the size of the megalosphere. If it is small the 

 arrangement approaches that of the microspheric form, if large it 

 departs more widely from it. 



Another feature met with in some of these stunted forms, 

 though by no means in all, is that the subdivision into chamber- 

 lets may be incomplete or wholly absent. Sometimes the sub- 

 division dies out in the terminal chambers after becoming estab- 

 lished in their predecessors ; in others it is absent throughout the 

 test. I am inclined to regard these latter forms as examples of 

 Orbiculina which have lost their secondary septation by "degenera- 

 tion " rather than as representatives of Peneroplis, because of the 

 existence of the intermediate forms just alluded to, in which the 

 subdivision dies out in the terminal chambers, and also because 

 they agree so closely in external features with small examples of 

 " typical " Orbiculina, that they cannot be distinguished from them 

 by the external characters of the tests. 1 



Four well-marked species are generally included in the genus 

 Orbitolites, of which three 0. marginalis, duplex, and complanata 

 are intimately related to one another, and form a remarkably 

 complete series -of grades of development, while 0. tenuissima 

 stands apart. The three former are inhabitants of the littoral 

 zone of tropical and subtropical seas, while the last lives in the 

 deeper parts (250-1700 fathoms) of the North Atlantic, from 

 which it extends into the Mediterranean. 



In all the annular arrangement of the chambers is assumed 

 early in life, the tests have a flattened discoidal shape, and an 

 umbo is absent, as the chambers are not equitant at any stage of 

 growth. All but the earliest chambers are subdivided into 

 chamberlets. 



In 0. marginalis (Lamk.) the chamberlets are generally some- 

 what quadrangular when seen on the face of the disc, and the 

 chambers they compose have an evenly curved outline. The 

 disc consists of a single layer of chambers, and they are throughout 

 simply applied to the peripheral margins of their predecessors. 

 The radial septa which divide the adjacent chamberlets of an 

 annulus from one another are traversed at their peripheral 

 border by a canal, which places the chamberlets in communication 

 with one another, and the canals of any one annulus may thus be 

 regarded (following Carpenter's nomenclature) as composing an 

 annular canal. From the canal, as it traverses a septum, a passage 

 leads in a radial direction and opens either to the exterior by a 



1 I have not had the opportunity of examining examples of Archiacina, -but from 

 the figure given by Schlumberger (50, Plate III. Fig. 2) it seems possible that this 

 may be a form of variety compressa which has similarly lost the subdivision of its 

 chambers. 



