REPORT ON THE HEXACTINI^ ' 27 



which renders the estabHshment of a final definition exceed- 

 ingly uncertain. 



The name Urticina was originally applied by Ehrenberg 

 ('34) to a subdivision of his subgenus Actinia Isacmsea, and in- 

 cluded numerous forms now assigned to other genera. It was 

 not until much later that the genus became at all definitely 

 limited and then it was under the name Tealia, proposed by 

 Gosse in 1858. The essential peculiarity of the genus accord- 

 ing to the definition given by Gosse, was that the verrucae were 

 scattered irregularly over the column wall and were not ar- 

 ranged in vertical series, and this supposed characteristic was gen- 

 erally accepted by succeeding authors. Messrs. G. Y. and A. F. 

 Dixon ('89) pointed out that this peculiarity does not really ex- 

 ist, the verrucas being really in vertical series, though the regu- 

 larity of the arrangement is not always readily perceivable, and 

 Carlgren ('93) has called attention to the same fact. The 

 original distinguishing peculiarity which separated the genus 

 from Cribrina being thus disposed of, both the Dixons and 

 Carlgren found a new distinction in the decamerous arrange- 

 ment of the mesenteries, the former authors, indeed, going so 

 far as to suggest that this peculiarity was worthy of being 

 raised to the dignity of a family characteristic. 



To establish a genus on its decamerism seems to me, in view 

 of what we now know concerning departures from hexamerism 

 in the Hexactiniae, to place it on an exceedingly insecure 

 foundation. And that this is true in the present case has been 

 recently shown by Verrill ('99, p. 216, note) who states that he 

 found '* many specimens [ of Urticina crassicornis] hexamerous 

 both as to tentacles and mesenteries ; many others decamerous ; 

 some octamerous ; and a few irregular or unequally developed 

 on opposite sides." A careful study of the mesenteries of the 

 individuals contained in the present collection reveals in no 

 case a perfect decamerism, but an irregular arrangement which 

 appears, however, to be based on a decamerism. Consequently 

 we may, I believe, hold the character of decamerism to be in- 

 sufficient for the characterization of the genus, and if it is to 

 be maintained distinct from Cribrina, we must seek for other 

 peculiarities. 



