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XVI. — Observations on Xanthidium, both Fossil and Recent. 

 By Samuel James Wilkinson, Esq. 



(Read January 14, 1846). 



The interesting genus Xanthidium among the fossil Infusoria is so 

 well known to microscopists that it is quite needless for me to enter 

 upon an explanation of its peculiarities. My object is to bring under 

 notice two peculiar forms of Xanthidium which I have lately met with 

 in some slides prepared from gun-flints, and which differ from any of 

 the figures accompanying Mr. H. H. White's paper on this subject 

 (see Vol. i.p. 77), — one in particular, as at Plate XIII. figure 1, where 

 it will be perceived that the tentacula, instead of diminishing in 

 diameter from the base towards the apex, gradually dilate in that 

 direction, a difference so remarkable that there can be no doubt of its 

 being a distinct species. Unfortunately, I am unable to state that 

 more than one specimen has come under my observation, and that 

 one has the disadvantage of being imperfect. Figure 1 represents it 

 as it appears under the microscope, but there is a sufficient portion 

 of it to form a very correct idea of its perfect form, of which the fol- 

 lowing may be taken as the characters. 



Body ovate, hollow, with tubiform tentacula, smallest at the base, 

 and gradually dilating to the apex, having their terminations depressed 

 and irregularly stellate. 



The body is smaller than in other fossil species when compared 

 with the tentacula, which latter are nearly equal to two diameters of 

 the former, and if the specimen were entire they would amount to from 

 twelve to fifteen in number. The size of this species is about ^-f^th of 

 an inch in diameter, and I beg leave to distinguish it under the name 

 of Xanthidium claviferum* from the nail-like form of the tentacula. 



The other fossil species, as represented in PI. XIII. fig. 2, is likewise 

 in a portion of a gun-flint, but I do not think there is sufficient diffe- 

 rence between it and Mr. H. H. White's figure 5 of PI. VIII. Vol. i., 

 which he calls Xanthidium spinosum, to designate it as new, and 



* This species was originally named Xanthidium tubiferum dilatatum, but as the 

 duplex trivial name is contrary to the rules of nomenclature, it has been thought 

 advisable to alter it to Xanthidium claviferum, which is perhaps more expressive of 

 the character of the tentacula. 



TRANS. MIC. SOC. VOL. II. I 



