PALEONTOLOGY. 9 1 



ZOANTHARIA RUGOSA, Milne-Edwards. 



So called in allusion to the radial rugae or plications of the stony, 

 calcareous polyp cells, which are the only part of these animals that 

 has been preserved. The Zoantharia apora of a similar structure 

 are distinguished from these by a difference in the arrangement of 

 the radial plications. Milne-Edwards assumes four primary plica- 

 tions in the cycle of radii in the Zoantharia rugosa, and six primary 

 plications in the Zoantharia apora, and deduces therefrom, in an 

 elaborate essay, a law of symmetry, according to which the mul- 

 tiplication of the radii in the cycles takes place, and by which he 

 endeavors to demonstrate that in case a new plication or lamella is 

 formed in the interstice between two older plications of a certain 

 value in the cyclical order, in all interstices, limited by plications 

 of the same value, the intercalation of a new lamella takes place 

 simultaneously, and this law he supposed to be governing the 

 growth of Zoantharia rugosa, as well as of the Zoantharia apora. 

 In the latter order this rule seems to be in force, but it does not 

 apply to the growth of the Zoantharia rugosa. 



The radial plications of the Zoantharia rugosa are arranged in 

 four primary fascicles, separated from each other by more or less 

 conspicuous gaps. These fascicles, apparently segments of a cycle 

 of rays, are in reality bilaterally situated in symmetrical position 

 on an axal line, dividing the apparent cycle in two halves. The 

 two fascicles on one side are equivalent to those of the opposite 

 side, but differ from one another. For better illustration, we may 

 compare the circumference of a polyp cell to a horseshoe with nar- 

 row, almost closed aperture. Opposite this aperture, in the centre 

 of the curve, two fascicles meet with their equivalent sides, leaving 

 an obscure, narrow gap between them, the centre of which often 

 exhibits a solitary, independent plication. This gap may, in distinc- 

 tion from the other gaps, be designated by the name of central gap. 

 At the ends of these fascicles, remote from the central gap, and 

 directed toward the aperture of the horseshoe, the plications be- 

 come gradually shorter, and, seen from the peripheral surface of the 

 polyp cells, do not extend to the apex of the conical polyparium, 

 but terminate above, nearer the calycinal margins. Another gap 



