INFUSORIA. 
55 
S. paradoxum, Meyen, (Nov. Act. Nat. Cur. Vol. 16, p. 777, PI. 43.) 
' “ Formed principally of two cells united end to end, and each terminated by cross-shaped pro¬ 
longations, on which are perceived vestiges of articulation.” (See Ferussac’s Bulletin, June, 1830. 
I am not sure that I have yet met with this species in America; I have, however, often 
seen the binary bodies represented by figs. 3 and 4, pi. 1, having four arms instead of the 
three represented in our drawing. In the four-armed state they agree closely with the above 
characters of paradoxum, as well as with Kiitzing’s figure 83. 
Pentasterias. Free, a simple univalve pentagonal carapace. 
Pentasterias margaritifera. Surface granulated, rays thick and obtuse. (Mandl and Ehrenberg, 1. c. 
PI. 8, fig. 46.) 
I am unacquainted with this genus, unless it is founded on five-rayed bodies resembling 
figure 7, which are only varieties of figures 3 and 4; the number of arms being, as I have 
repeatedly seen, liable to much variation. 
Tessararthra. Free, a simple carapace, univalve, globular, smooth, forming chains of four or more 
individuals by spontaneous division. 
Tessararthra moniliformis. Corpuscles green, two or four united in a right line. (M. and E., I. c. 
PI. 8, fig. 47.) 
I have not noticed this genus in America. 
Sph.®;rastrum. Free, a simple carapace, univalve, smooth, inflated, forming groups of various forms 
by imperfect spontaneous division. 
I have not yet detected any specimens of this genus. 
Xanthidium. Free, a simple carapace, univalve, globular, bristling with points or seta, isolated, 
binary or quaternary, [catenate ?) 
This genus is very interesting from the fact that bodies, almost identical in form with the 
living species, occur abundantly, preserved in the fossil state in flint. Drawings of several 
fossil species may be seen in the Annals of Natural History, PI. 9, accompanying an interest¬ 
ing paper on the Organic Remains in the Flint or Chalk, by the Rev. J. B. Reade. The di¬ 
rections given for finding these bodies in flint, are “ to chip off thin fragments which may be 
attached by means of Canada balsam to slips of glass, and then coated on the outer surface 
with hard spirit varnish. A hundred specimens may be thus cut, polished and mounted for 
the microscope without trouble and expense, and in less time than an expert lapidary could 
prepare a single slice with the diamond mill and polishing tool.” Many of the common gun 
flints contain these bodies. I am indebted to E. J. Quekett, Esq. of London, for very fine 
specimens of fossil Xanthidia; among them is a slice of flint prepared by a lapidary, which 
contains in a space of less than a square inch, as many as eight or ten very perfect Xanthidia, 
a scale of a fish, and other organic bodies. 
There has been much discussion as to the real nature of the bodies in flint, which so closely 
resemble the recent Xanthidia; Turpin mistook them for eggs of Cristatella, to which, how¬ 
ever, they have but little resemblance. Ehrenberg gives figures in his small treatise, “Die 
