188 



Wer die zweite Alternative bejaht, bringt damit zum Ausdruck, daß er 

 einen Theil der Übereinstimmungen mehr als Producte paralleler 

 Züchtung, denn als homophyletische Bildungen auffaßt. Aber auch 

 er wird anerkennen müssen, daß jene Züchtung auf einer sehr ähn- 

 lichen, weil nahe verwandten, Grundlage stattgefunden hat, und daß 

 ein anderer, kaum minder bedeutsamer Theil von Übereinstimmungen 

 ofifenbar homophyletischer Natur ist. 



4. An Abnormal Clypeastroid Echinoid. 



By W. L. Tower. 

 (With 3 figs.) 



eingeg 27. Februar 1901. 



Specimens of Echini exhibiting abnormalities in the number of 

 ambulacra or inter-ambulacra are certainly not frequent, if the fifteen 

 or twenty known cases are any indication of scarcity. Every year in 

 the laboratories of Europe and America many hundreds of specimens 

 of various species of sea-urchins are examined, yet only rarely are 

 there found any variations excepting those of minor structures, as the 

 apical plates, the rows of plates in the inter-ambulacra, etc. 



In a lot of about two-hundred specimens oî Echinarachnius parma, 

 from Wood's Holl, Mass., one abnormal individual was found. A 

 careful examination of this material as well as of all other of this 

 species that I could get failed to show any specimens like the unique 

 one here described or any other variations of any moment. Prof. A. E. 

 Verrill tells me he has examined many specimens of this species and 

 has never seen any variation in the number of ambulacra or inter- 

 ambulacra. Prof. R. T. Jackson likewise informs me that he has 

 not observed any similar case in the large amount of material of this 

 species which has come under his observation. Hence the variation 

 exhibited by this specimen must certainly be a rare one, and inter- 

 esting as being the first instance of a variation of this class in this 

 genus of the Clypeastroid Echini. The known cases of these variations 

 in Echini, as far as I can determine them from the literature at my 

 command, are distributed as follows in eight genera: Arhacia (1), 

 Cidarites ( 1), Echinoconus { 1), Discoidea (1), Echinus (2), Amblypneustes{4:), 

 Hemiaster (6), Echinar achnius (I). 



When discovered the specimen was dry and almost devoid of its 

 spines, consequently, no study of the soft parts could be made. It is 

 probable that as these were so closely correlated in position and struc- 

 ture with the hard parts they also were abnormal. The specimen was 

 treated with potash (KOH) to bring out the sutures between the 



