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Show 10 PROF. E. A. MINCHIN ON THE [May 2, of view of substantive variations of form and size; secondly, as regards numerical variation, that is to say abundance of monaxons compared with other types of spicule. The monaxons are all of large size, being at least twice as thick as the basal rays of the triradiate systems, and not less than 300 fi in length, allowing for those which are apparently not full-grown. But in some specimens the monaxons reach a size which can only be called gigantic. In a specimen from Banyuls sent me by Topsent (which I will refer to as Topsent 12 e), the monaxons, when drawn to the same scale as the other spicules figured here, come out 32 centimetres in length, corresponding to an actual length exceeding 1000 fi (1 mm.), with a breadth of about 50 /j. at the thickest part. Even these proportions are exceeded by a specimen in my collection from Banyuls, in which the monaxons when drawn to scale measure 75 centimetres in length, corresponding to an actual length of 2343 fi (2‘3 mm.). I do not think that spicules of such size have been recorded from any Ascon. The large monaxons of Ascandra densa and A. parus figured by Haeckel {I. c. pi. 14. figg. 2 c, 3 f ) fall far below those that I have mentioned in dimensions. With these extraordinary variations in size, the form and characters of the monaxons are fairly constant (text-figg. 2 and 3, 2 h, 3 c-3 h). They are spindle-shaped, pointed at both ends, slightly curved, sometimes distinctly so when more slender, or nearly straight when very thick. There is no lancet-head present at the distal extremity, as figured by Haeckel; his figure {I.e. pi. 14. figg. 6 d, 6 e) almost certainly refers to complicata (compare his figg. 1 g- 1 Jc, on pi. 15). It is, indeed, impossible to say wdiich is the distal end of these monaxons, as they do not project from the sponge like the true (primary) monaxons of other Ascons. Near the middle of the spicule, sometimes at about one-third of the length from one end, a slight constriction can be observed, sometimes very distinct, in others very shallow, in others again represented by an annular thickening, and sometimes not to be made out at all. This constriction is more distinct in young spicules, and appears to become more or less obliterated with growth. In big spicules the contours are often so sinuous and irregular that the primary constriction may be masked by secondary curves. I consider this primary constriction, as I propose to call it, of great morphological importance, as indicating probably that these spicules are not primary monaxons *, comparable to those of Leucosolenia complicata, for example, but in reality derived from a triradiate by loss of one ray and shifting of the two others into approximately the same straight line. In very young monaxons of contorta I have noticed a delicate transverse line in the region of the constriction (text-fig. 3, 3 e), and I have also found a spicule of which it would be difficult to affirm whether it is a young * A primary monaxon is derived from a single mother cell which divides into two triradiate 8°stem onSmahnS m ex™tly the same manner as a single ray of a |