GENUS DACTYLOPORA. 133 



a different category. A completely developed specimen of this organism is usually rather 

 barrel-shaped than cylindrical ; and it is in the more attenuated varieties (on one of 

 which, presenting a difference of superficial characters that will be shown to depend merely 

 upon the degree of attrition to which the specimen has been subjected, Defrance founded his 

 genus Poliitri/pe) that the cylindrical form is most pronounced. The specimen of I), ci/lin- 

 dracea figured by Defrance (xxix, ' Zooph.,' pi. xlvii, fig. 4) measures nearly half an inch 

 in length, and one sixth of an incli in diameter at its largest part ; but the largest specimens 

 which have fallen under our observation are by no means of these dimensions ; and we have some 

 that are quite perfect as to form, the length of which does not exceed one sixteenth of an inch. 

 The extremities are rounded off by an inflection of the wall of the cylinder towards its axis, 

 so as partly to close-in the cavity ; there always remains, however, a large central orifice, at 

 least at one termination ; but whether this is maintained throughout life at both ends we are 

 not able to speak positively, the figures of Defrance indicating a complete closure at one end, 

 which none of our specimens exhibit. Notwithstanding the very considerable diversities in 

 internal structure which (as will presently appear) are exhibited by different varieties of this 

 organism, the external surface presents the same characters in all ; being covered with a series 

 of somewhat funnel-shaped cups, at the bottom of every one of which (usually, but not 

 always, in the centre) is a circular pore (Plate X, figs. 20 a, 29,(7, .</)• The superficial edges of the 

 dividing partitions between these cups, in the specimens which show least evidence of having 

 been altered by attrition, are roughened by a sort of rude sculpture, which reminds us of that 

 of the outer surface of the chamber-walls of Z). c/landidosa (fig. 27) ; and this circumstance seems 

 to indicate that we have here the normal exterior of the organism, and not (as was supposed 

 by Messrs. Parker and Rupert Jones, lxxix) a surface produced by the wearing-away of the 

 outer walls of cavities that were originally closed or nearly so. The pores in these cups are 

 the orifices of a system of radiating passages that constitutes the peripheral continuation of the 

 "junctural interspaces;" and it is in the extraordinary development of this "interspace-sys- 

 tem," and in the formation of a sheath of solid shell-substance (often of considerable thickness) 

 around the chambered cylinders, that the distinguishing peculiarity of D. ct/lindracea consists. 

 The degree of development of this solid sheath, and that of the " interspace-system," vary 

 greatly in relation to that of the chambered cylinder ; but they always correspond so closely 

 the one to the other, that it is impossible not to recognise in this " interspace-system" the 

 representative of the " canal-system" of the larger hyaline Foraminifera, and in the solid 

 sheath the analogue of their " intermediate" or "supplemental skeleton " (1 63). This dif- 

 ference, however, although one that greatly modifies the aspect of the walls of the cylinder as 

 they show themselves in transverse section, is not really of so much account as a very remark- 

 able diversity in the relations between the " interspace-system " and tlie cliambers ; a diversity 

 that seems at first sight of so fundamental a nature, as to justify the belief that the specimens 

 which exhibit it must be formed on plans which are essentially unlike. I have succeeded, 

 however, in tracing these differences so gradationally, not only through an extensive series of 

 distinct specimens, but through the different parts even of one and the same, that I can enter- 

 tain no doubt of their being all varietal modifications of a common type ; and much of the 

 perplexity at first caused by them disappears, when it is remembered that the " interspace 

 system " is not a definite fundamental part of the organism, but that it is an indefinite residue 

 (so to speak) left by the non-consolidation of certain passages which seem to be left for the 



