TAKIETy or ALECTONA MILLARI. 537 



Bearing in mind the difference in habit between the latter and 

 the shell-boring form, the difference in the localities, and the 

 freedom of both from foreign bodies generally, this association of 

 such widely different spicules seems especially difficult to under- 

 stand. Thinking that the determination of the species of Disco- 

 dermia from which the spicules are derived and a comparison of 

 its habit and distribution might lead to some explanation, I have 

 endeavoured to find a record of such a form. 



The spicules are perfectly constant in form and size ; in shape 

 triangular, with rounded angles and measuring '15 millim. in 

 diameter. The three canals in the disk ai-e well-marked, and the 

 rudimentary ray short, conical, and pointed. Over the surface 

 are scattered circular j)apillse. 



The margin is always entire, never lobed — a character of the 

 young dermal spicules in many Lithistids, but not a constant one 

 in any known species. 



The nearest form seems to be the Discodermia papillata, 

 recorded by Mr. Carter from the Grulf of Manaar *. In this case 

 the papillae are very much smaller and more numerous, while all 

 except the youngest dermal spicules are lobed. 



As the species was encrusting and perhaps partly excavating a 

 Melohesia, the description suggested the possibility of the pre- 

 sence of an excavating Discodermia in the shell of Lima, subse- 

 quently replaced by Alectona. If such were the case, however, 

 the skeleton-spicules of the Lithistid would be found as well. 

 The fact that only dermal spicules occur, indicates that they have 

 been derived from some distance, as they are not only easily de- 

 tached, but also easily conveyed by currents. 



Systematic Positio7iof the ffemis Alectona,. — In conclusion, it may 

 be as well to point out that no satisfactory position in classifi- 

 cation has been found for the genus Alectona, and that the 

 nature of its spicules prevents its inclusion in any of the groups 

 of the Monaxonida as defined in recent systems. 



Mr. Carter placed it witli other boring sponges in his division 

 "JEccoelonida "t, but there is no doubt that a classification founded 

 on a single character must give way to one with a morphological 

 basis. 



In Dr. Vosmaer's arrangement J the genus is placed after 

 Cliona, but it is only added in the appendix. 



* Ann. & Mag. Nat. Hist. (5) vol. vi. 1880, p. 146. i" Ibid, p .58, 



'^ Bronn, Klassen u, Ordnmigen des Thierreicbs — Porifera, p. 406. 



