88 FAMILY MILIOLTDA. 



uncommon to meet with individuals which present i\ combination of the characters of both, 

 the septal plane having a single row of pores in one part of its length with a double row in 

 another (sec ex, fig. 85). Sometimes, moreover, in one of the less compressed forms of the 

 shell, although there is but a single row of pores, it is obvious from their elongated shape 

 that there is an incipient tendency to duphcation (fig. 8). Hence I consider that it may be 

 unhesitatingly asserted that the duplication of the row of pores, and the increased turgidity of 

 the spire which it accompanies, are but features of individual variation, and cannot be 

 admitted to rank as specific differences ; and in this view I am glad to find myself borne out 

 by Prof. Williamson, who (ex, p. 44) defines Peneroplis, not (like M. D'Orbigny) as 

 having only a single row of apertures, but as having " septal orifices scattered over the long, 

 narrow, septal plane." 



127. A much wider departure from tlie typical PcncfopJls, however, is presented by that 

 group of forms which D'Orbigny has separated under the generic designation iJraiJ^nYzwa; 

 and according to the usually received notions on the classification of Foraminifera, such a 

 separation would seem fully justifiable. These forms are especially characterised by the 

 possession of a single large apei'ture, sending out dendritic ramifications, in each septum 

 (Plate VII, figs. 1, 14) ; but this is by no means the whole of their differentiation. For the 

 spire, instead of being compressed, is very turgid (fig. 13); and its successive whorls not 

 merely surround those which have preceded them, but also invest them with broad alar pro- 

 longations {al, fig. 14, h, (■), those of the chambers of even the last whorl often extending 

 neai'ly to the umbilicus. The geographical distribution of Dnidritina, moreover, is peculiar ; 

 for, so far as I am aware, this type is restricted to tlie tropical ocean. I have not met with 

 it in dredgings from any part of the Mediterranean or the Red Sea, where Peneroplis 

 abounds ; while the largest specimens I have seen are those furnished by Mr. Cuming's 

 Philippine explorations, some of which measure '078 inch in diameter, and '030 between the 

 lateral surfaces. To such as content themselves with glancing at strongly marked examples 

 of this type, the propriety of its generic, or at any rate of its sub-generic, separation from 

 Peneroplis would seem indubitable. Nevertheless I think that I shall be able to show 

 adequate grounds for the belief that the two forms cannot be separated by any definite line of 

 demarcation, and that they must therefore be ranked as not merely belonging to the same 

 genus, but even as varieties of the same species. 



128. In the first place, I would refer to the fact that the peculiar plication and puncta- 

 tion of the surface of the shell, which are such marked features in the physiognomy of 

 Peneroplis, are repeated in Dendritina (fig. 21) in a manner so precisely similar, as strongly 

 to impress every one who has his attention directed to the aspects of these two forms 

 respectively with the idea of their very close relationship. Secondly, we observe in 

 Dendritina precisely the same tendency to rectilineal extension in the later period of growth 

 as in Peneroplis; for although the distinct generic term Spirolina has been given to the 

 form presenting this modification, it is obvious that the example delineated in fig. 4 

 bears just the same relation to the typical Dendritina, that the one shown in fig. 12 bears 

 to the typical Peneroplis. The transition from Dendritina to SjnroHna is well seen in fig. 13, 

 in which we see that the last whorl is just about to disengage itself from the earlier ones. 



