92 



LOWER PENINSULA. 



separates these shorter plications on each side from the joining 

 fascicles of plications, which extend to the ends of the arms of the 

 horseshoe. This pair of gaps are the lateral gaps. The further ends 

 of this second pair of fascicles approach each other again, in the 

 aperture of the horseshoe, leaving another larger gap between 

 themselves than the other fascicles, which may be termed apcrtural 

 gap ; its centre is, like the opposite obscure gap, occupied by a 

 solitary plication. The plications of this second pair of fascicles 

 are longest and extend to the apex of the polyparium on their end 

 joining the lateral gaps, and shortest at the apertural gap. This 

 is the order in the structure of all the polyp cells of the Zoantharia 

 rugosa. If, during the progress of growth, new'plications are added 

 to the cycle of existing ones, the new ones are only inserted at 

 those ends of the four fascicles which are directed toward the 

 apertural gap, while the already existing plications are never dis- 

 turbed by interposition of new ones, excepting, as indicated at the 

 four ends of the fascicles, directed to the apertural gap ; further- 

 more, the addition of new plications at the four ends of the fascicles 

 ^snot always contemporaneous in all, or in the opposite correspond- 

 ing ones, for otherwise the lamellae in each equivalent bundle should 

 be equal in number, which is not always the case. This bilateral 

 structure of the polyp cells of the Zoantharia rugosa has been 

 observed by several palaeontologists, and been mentioned by them 

 as a peculiarity of certain species ; but the late Dr. Kunth, of Berlin, 

 was the first to demonstrate this bilaterality to be an essential char- 

 acter of all the Zoantharia rugosa, and to exhibit with clearness the 

 peculiar mode of multiplication of the lamellae in this order. If we 

 examine a Streptelasma or a Zaphrentis, we find the outer surface 

 of the polyp cells longitudinally striate, by broad, convex bands or 

 ribs, and by intermediate, narrow, linear furrows. The furrows cor- 

 respond to the crest-like plications on the inside of the calyces, the 

 ribs to the interstitial spaces between them. Three of such longi- 

 tudinal furrows are, on each of the polyp cells, more conspicuous 

 than the others ; they correspond to the gaps between the bundles 

 of lamellae. In the furrow corresponding to the apertural gap, the 

 other furrows from both sides converge at acute angles, like the 

 barbs of a plume, to its keel, gradually becoming shorter as they 

 approach the margins of the calyx. The two other obvious fur- 

 rows, corresponding with the lateral gaps, arc, on the side nearest 



