356 Fisher — Disintegration of a Chalk Cliff. 



CPxQq = TPxPp. 

 or, if A M = X, M P = y 



(a— y) d X = ^-j^ X P m cos a 



= ilEa (*^ y — ^ ^ t^'i ") cos a 

 = y dy cot a — y d x 

 or a d X =: y dy cot a 



Integrating, and observing that x and y begin together — 

 a X = V y2 cot a 



"2" 



or y^ == 2 a tan a x 

 ■whicb is the equation to a parabola, of which the vertex is A, and 

 the latus rectum is 2 a tan a. The proportions of the curve will 

 therefore be increased by an increase in the height of the cliff, or by 

 an increased capacity in the talvTS to stand at a high angle. 

 We have ^y _ _a ^^^^^ 



dx y 



= tan a when y = a 



Hence we have the very elegant result that the curve vnll be con- 

 tinued unbroken to the very top of the cliff, at which point its in- 

 clination to the horizon will be identical with that of the talus. 



Y. — Some Observations on the Zoantharia Rugosa} 



By GUSTAVE LiNDSTKOM, Ph.M. 



[PLATE XIV.] 



PEOFESSOE STEENSTEUP,^ some years ago, questioned the 

 fact as to whether the Zoantharia tahulata and rugosa, included 

 by him under the common name of " Cyathophylla," might be con- 

 sidered as true polyps. MM. Edwards and Haime in framing those 

 great subdivisions of their " Coralliarta," remarked their striking 

 dissimilarity from the other Actinozoa. Professor Agassiz, in his 

 grand monograph on the Acalephce of North America,^ considers 

 these differences so important that henceforth all connection be- 

 tween the above-named groups and the Zoantharia aporosa and 

 perforata will be impossible. But besides these peculiar cha- 

 racteristics ofi the Rugosa, such, for instance, as the compact imper- 

 forate structure of the calyx and septa (the septa originating from 

 four primary ones), the absence of costas, the strange septal fossEe 

 in the bottom of the calyx, the processes resembling rootlets, the 

 transverse floors or tabulse in the interior, which often have a 



1 Translated by the Author, fi-om the original Swedish — " Ofversigt af Yetens- 

 kaps Academiens Forhandlingar." 



- On the Systematic Place of the BracMopoda and the CyatJiophylla (in Danish), p. 

 20, 1843. 



2 Contrib. to the Nat. Hist, of the United States. Vol. iii. p. 121. 



