538 ON A YARIETT OP ALECTONA MILLABI. 



Messrs. Eidley and Dendy *, and Dr. E. von Lendenfeld t, 

 have classified the Monaxonida according to their spicular 

 characters, but neither arrangement includes the genus under 

 consideration. In their division of the ' Clavulina ' are included 

 forms with microsclera of the "stellate " type j and with monactinal 

 megasclera. 



If, as the authors of the ' Challenger' Eeport on the Monaxonida 

 are of opinion, the microsclera are of more classificatory value 

 than the megasclera, it may be possible to place Alectona among 

 the SpirastrelUdcB near to Latrunculia, in spite of its diactinal 

 spicules. Two sj)ecies with oxeate skeleton-spicules have indeed 

 been included in the latter genus by Mr. Carter §, and Messrs. 

 E/idley and Dendy have described as Latrunculia (?) acerata a 

 third species with similar megasclera associated with microsclera 

 of the " sceptrella " form. 



If such a form as this can be retained among the Clavulina, 

 the genus Alectona may be placed also in that division ; but it 

 will possibly be found advisable to establish a new group for the 

 reception of those Monaxonids that have oxeate megasclera and 

 stellate diactinal microsclera. Moreover, if, as the spiculation of 

 the original specimen of A. Millari seems to indicate, the scep- 

 trella can be derived from a simple oxea by development of 

 lateral scattered projections and their gradual localization round 

 two centres, the classificatory value of stellate microscleres may 

 have to be reconsidered. 



EXPLANATION OF PLATE XIII. 



Fig. 1. View of the inside of the shell of Lima excavata, Fabr., attacked by 



Alectona Millari. Nat. size. 

 Fig. 2. Outside of the same shell. Nat. size. 



Figs. 3, 4. Ends of two passages opening on the surface of the shell, x 16. 

 Fig. 5. Part of the spicular crust round the opening of one of the same. X 50. 



* ' Challenger ' Eeports, toL xx. pp. liii-lxviii. 



t Proc. Zool. Soc. 1887, pp. 658 662. 



X For spicules of the shape of the microscleres of Alectona, Mr. Carter pro- 

 posed the name of "seeprrefe." Messrs. Eidley and Dendy propose the term 

 " discastra" as a synonym (' Challenger ' Reports, Zool. vol. xx. p. 263), but 

 the forms to which they apply it are monactinal. It would perhaps be useful 

 to keep Mr. Carter's term for diactinal forms, and use discastra for monactinal 

 spicules like those of Latrunculia. 



§ Ann. & Mag. Nat. Hist. vol. iii. 1879, p. 298 ; vol. vii. 1881, p. 380. 



