by the very concave, almost campanulate disc, formed of arched 
cells. ‘lhe coherence between the cells also appears to be less 
strong than in the other species; but there is no difference in 
the stem, which is equally furnished in all with whorls of dot- 
like scars at intervals. Although the byssoid fibres have not 
been yet observed in 4. Calyculus, there can be little doubt that 
the scars denote the spots from which fibres have fallen. 
Mr. Clifton’s specimens were attached to the valves of bivalve 
shells. 
Fig. 1. Group of ACETABULARIA CALYCULUS, on a piece of shell,—the natural 
size. 2. Upper part of a frond. 3. One of the radiating cells of the disc, 
in its mature state, full of spores. 4. Spores :—the latter figures magnified. 
