﻿78 



Proceedings of the Royal Physical Society. 



cent, have less. Further, twelve, thirteen, and fourteen-rayed 

 animals account for 94 per cent, of the total. 



The accompanying " Polygon of Frequency " diagram shows 

 very clearly the prevalence of thirteen-rayed Solasters. A 

 curve, which is a simple uni-modal one (Pearson's Type IV.), 

 constructed from these ficpures after the manner of writers in 

 Bionutrika, would portray the facts even more graphically, 

 and would involve the following factors : — 



Mean, .... 



Standard Deviation, 



Coefficient of Yariabilitv, 



Probable Error of Mean, 



Probable Error of Standard Deviation, 



Average Deviation, 



Asymmetry, 



= 12-7827 

 = 0-8024 

 = 6-2771 

 = ±0-02187 

 = ±0-015i6 

 = 0-5896 

 = 0-1468 



V. On the Occurrence of a, Supposed Australasian Hydroid 

 (Sertularia elongata) in the North Sea. By James 

 EiTCHiE, M.A., B.Sc, University of Aberdeen. [Plate 



III.] 



(Read 28tli Januaiy 1907.) 



In ISTovember 1904, Mr J. J. Simpson, M.A., B.Sc, while 

 searching for chance specimens in the nets of the trawlers 

 lying at Aberdeen Fish Market, came upon a magnificent 

 cluster of Hydroid colonies, which he kindly handed to me 

 for identification. 



The colonies, of which there were about forty-five in the 

 bunch, are fixed upon a slender twig, 0"2 mm. in diameter 

 and 11 cm. in length, which is in many places encrusted by 

 Polyzoa. They vary considerably in size, the largest about 

 8 cm. in height and 1*5 cm. in breadth, the smallest about 

 2 cm. by 0'45 cm. When Mr Simpson found the specimens, 

 the coenosarc had already wholly disappeared. Both the 

 hydrocaulus and the gonothecee (which are numerous, 

 some of the colonies bearing a gonotheca on almost every 

 pinna) agree specifically with the descriptions and figures 

 of Sertidaria elongata, Lanix., given by Prof. D'Arcy W. 



