92 Mr Lister, On the relation in size between the 



On the relation in size between the megalosphere and the 

 microspheric and megalospheric tests in the Nummulites. By 

 J. J. Lister, M.A., F.R.S., St John's College. 



[Read 13 March 1905.] 



At the meeting of the Society on 31 October, 1904, the author 

 drew attention to the fact that in the three English species of 

 Nummidites, Viz. N. laevigatus, variolarius and ' elegans,' both 

 megalospheric and microspheric forms were represented and 

 associated in the Bracklesham and Barton beds of the Hamp- 

 shire basin. 



A comparison of the sizes of the megalospheres in these species 

 suggested that a definite relation might exist between them and 

 the sizes of the whole microspheric tests. 



To examine this question the following species have been 

 examined : Nummulites complanatus (tchihatcheffi), N. perforatus 

 (Lucasanus), N. gizehensis (curvispirus), N. perforatus var. obesus 

 (Lucasanus var. obsoletus), N. laevigatus (Lamarcki), Assilina 

 exponens (mamillata), Nummidites biarritzensis (guettardi), N. dis- 

 corbinus (sub-discorbinus), N. Orbignyi (wemmelensis var. elegans) 

 and N. variolarius (Heberti). (The name included in brackets is 

 that of the alternate form of the species. In all cases except the 

 last it happens that the name of the microspheric form has priority 

 over that of the megalospheric.) 



The species are here arranged in order of the sizes of the 

 megalospheres, and this is found to coincide with the order of the 

 volumes of the microspheric tests (with the exception of the 

 variety obesus of N. perforatus, the microspheric test of which 

 falls one place out in the series). 



Polystomella crispa is the only member of the Nummulitidae 

 of which the details of the life-history are approximately known, 

 and in this the only mode of origin of the megalospheric form 

 which has been observed is, by an asexual process of reproduction, 

 from a microspheric parent. Hence it would appear that in this 

 mode of reproduction, in the Nummulitidae, the size of the offspring 

 is approximately proportional to the volume of the protoplasm of 

 the parent. 



