ORIGINAL COMMUNICATIONS. 
Description of a New Form of Naxrp-ryvep Mepvusa (Thau- 1 
mantias achroa), with Histological Details. By T. SpeNcER 
Cossoxp, M.D., F.L.S., Lecturer on Botany at St. Mary’s 
Hospital Medical School, London. 
(Communicated to Linnean Society, March, 1857.) 
On the 13th of August, 1856, after filling a marine aqua- 
rium, containing about three gallons of water, I detected a 
small naked-eyed Medusa swimming vigorously near the sur- 
face. This minute creature was immediately made the subject 
of careful study, and it turned out to be a species of Thau- 
mantias new to Britam (fig. 1, Pl. 1). So far as I can ascer- 
tain, it has not been described by any author abroad; I have 
therefore thought it worthy of a separate notice. The water 
containing the Medusa had been procured four days pre- 
viously from the shore near Leith, and had been kept in an 
earthen vessel hermetically closed. The animal was only 
preserved alive until the 16th of the same month, in conse- 
quence of injuries sustained during a prolonged and often- 
repeated microscopic examination. 
Referring to the various organs in detail, I allude in the 
first place to the umbrella. The form and general aspect of 
this structure resembles that of the more typical species, being 
hemispherical, transparent, colourless, smooth, slightly elon- 
gated vertically when in a state of rest, the transverse diame- 
ter measuring rather more than the third of an inch, and 
becoming much increased during contraction, the length of 
the disc, at the same time, being proportionately lessened. 
To remark particularly on such admeasurements may appear 
triflimg, but it is useful as an aid to diagnosis, because the 
animal bears a very marked resemblance to Thaumantias 
punctata and T. Thomsoni. These, however, present a more 
depressed umbrella. Again, it is similar, at first sight, to 
T. convexa ; but this species has the umbrella more cylindrical, 
and there are other distinguishing characters, which will be 
alluded to presently. The circumferential portion of the um- 
brella is frmged by twenty-four tentacula of extreme delicacy 
and unusual length ; also, by eight ocelli, a circular gastro- 
vascular canal, and a well-defined shelf-like veil directed 
inwards. These parts will be considered separately. Under 
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