594 TRANSACTIONS OF SECTION D. 



prominently present in full-grown specimens of this generation, although the 

 sexual nuclei are not formed until the next or megalospheric generation has 

 reached maturity. It would appear, therefore, that in Polystomella the chromidia 

 are associated with the formation of the nuclei of the reproductive elements, 

 ■whether these do or do not engage in conjugation. 



Goldsehmidt,^ in a very capable review of our knowledge of chromidia, is 

 inclined, on the ground of the apparent difference in relation to the life of the 

 organism between the structures so called by Hertwig, in Actinosphmrium, and the 

 chromidial nets and strands of Rhizopods, to the view that two physiologically 

 distinct elements have assumed a morphological similarity and mode of origin. 

 "WTiile, retaining the name chromidia for the former, he distinguishes the latter 

 under the name Sporetia. It is, however, perhaps somewhat early at present 

 to insist on this distinction. Hertwig's essay has already been most fruitful 

 in results, and we cannot doubt that the nature of the chromidia will be further 

 elucidated now that attention has been directed to them. 



The relation in size between the microspherie parent and the members of the 



megalospheric brood. 



There is one other point to which, before concluding, I wish to invite your 

 attention. 



In the course of the discussions on the significance of the occurrence of 

 nummulites in pairs, objection was taken to the view that the members of the 

 pair belonged to the same species on the ground that solitary forms — megalo- 

 spheric or microspheric, unaccompanied by the usually associated sister form — 

 occurred in certain localities. De la Harpe himself, having at first urged this 

 objection, withdrew it ; but it is still entertained by some palaeontologists, and made 

 the ground for maintaining the view that the members of a pair are specifically 

 distinct. 



On looking into the matter I found that two out of the three species of 

 Nummuhtes which occur in the Bracklesham and Barton beds in the Hampshire 

 basin were only known, so far as published descriptions went, in the megalo- 

 spheric form, although the corresponding microspheric forms had been found asso- 

 ciated with these megalospheric forms on the Continent. It therefore seemed worth 

 while to examine the English beds to see whether they might lend any support to 

 the view I have mentioned. The three English species are the following : — 



Nummulites Iff.vigatus (Brug.), megalospheric form ' N. Lamarcki, d'Arch.' 

 N. variolarius (Lamk.), microscopic form ' N. Heberti. d'Arch.' 

 and N. Orbignyi (Galeotti), megalospheric form 'N. zuemmelends, d. 1. H. and 

 V. d. Br., var. elegans, Sow.' 



In N. Icevigatus the microspheric form far exceeds the megalospheric, in the 

 size attained by the fuU-gi'own tests, as we have seen to be usually the case with 

 nummulites ; but in the other two species the size attained by the two forms is 

 approximately the same. Hence there is in them no external indication of 

 dimorphism, and it is necessary to grind down the little shells to expose the initial 

 chambers in section before they can be referred to one form or the other. The 

 results of the investigation are fully set forth elsewhere,' and I need only say 

 here that on proceeding in this manner with these two species, after grinding 

 down a number of examples which proved to belong to the commoner megalo- 

 spheric form, I came in eacli, as I fully expected I should, on examples of the 

 microspheric forms. The English beds, therefore, offer no support to the view that 

 one or other of the forms of a species may occur solitary.^ 



' ' Die Chromidien der Protozoen,' Arch.f. Protistenhmde, Bd. v., 1 (1904), p. 126. 



* Paper by the author ' On the Dimorphism of the English Species of Nummu- 

 lites and the Size of the Megalosphere in relation to that of the Microspheric and 

 Megalospheric Tests in this Genus.' Pro. Roy. Soc, vol. B. 76, 1905, p. 298. 



^ When the two forms are of different sizes, and the materials of a bed have been 

 rearranged by currents, they may, of course, be differently distributed. 



