108 THE VOYAGE OF H.M.S. CHALLENGER. 



Subgenus Homceodictya, Ehlers (Pis. XXIL, XXIIL, XXIY., XXIX.). 



1797. Spongia (pars), Esper, Die I'flanzenthiere, Fortsetz. 1 p. 190, pL 1. 

 1842. HaHcho7idna {parit), Johnston, British Sponges, p. 92, pi. ii. 



1866. Isodidya (pars), Bowerbank, Mon. Brit. Spong., vol. ii. p. 311 ; vol. iii. pi. lii. 



1867. „ (pare), Gray, Proc. Zool. Soc. Lond., May 1867, p. 534. 

 1870. Hormjeodietya, Ehlers, Die Esper'schen Spongien, Eriangen, 1870, p. 17. 

 1882. Ghalina {pars), Carter, Ann. and Mag. Nat. Hist., ser. 5, vol. x. p. 111. 



Form various, usually lobate or palmate. Megasclera oxea. Microsclera isochelse, 

 of a very peculiar type. The latter are the characteristic spicules of the subgenus ; the 

 shaft is slightly curved and may be expanded all the way along, from end to end, into 

 continuous, dehcate, lateral fiinbria3, which may be more or less curved ; in other words, 

 the two lateral palms on each side of the spicule may remain united together, and form 

 merely terminal portions of a continuous lateral fimbria. The anterior palm of the 

 spicule is distinct and somewhat oval in shape, usually slightly curved outwards at the 

 free end ; from the median line of the posterior surface of each anterior palm there 

 projects backwards, i.e., towards the shaft, a delicate, flat fimbria, pear-shaped in out- 

 line ; this gives to the anterior palm the appearance of being forked when the spicule 

 is viewed laterally, and is the distinguishing feature of the subgenus {vide PL XXIX. 

 fig. 7a). 



Although of a very peculiar form, the isochela of Homceodictya does not really 

 differ so much from an ordinary isochela as might at first sight be expected. We have 

 only to cut away the central portion of each lateral fimbria (in those cases where it 

 persists), leaving the ends as lateral palms (connected, of course, as usual, with the shaft 

 for their whole length), and to abolish the pear-shaped process which remains adherent 

 to the back of the anterior palm, and probably represents the remains of a connection 

 between the latter and the shaft, and we have left a typical palmate isochela, like that, 

 for example, of Desmacidon compressa.^ 



The history of this interesting subgenus is not a little involved, and wdl give some 

 idea of the literary difficulties which the spongologist has to encounter. 



Johnston, in his British Sponges (p. 92, pi. ii.) describes and figures a sponge under 

 the name Halichondria palmata, and gives in his list of synonyms, " Spongia palmata, 

 Sihh. Scot. ill. ii. 'lib. iv. 55, &c." With some difficulty we found the book thus 

 referred to,^ but the only reference we found to sponges consists of the two words 

 " Spongia palmata;" and we can hardly retain this as a synonym on so little evidence. 



Bow^erbank, in his Monograph of British Spongiada3 {loc. cit.), describes and figures 

 his Isodictya p)almata, which he identifies with Halichondria pahnata, Johnston. He 



1 Cf. also the descriptions given by Mr. Carter of the isochela oiHcmueodictya (Chalina) palmata and of Desmacidon 

 cpmpressa, Ann. and Mag. Nat. Hist., ser. 5, vol. x. p. Ill, fig. 1, a and b, and p. 113. 



2 Nuncius Scoto-Britannus, sive Admonitio de Atlante Scotico &c., by Sir Robert Sibbald, Edinburgh, 1683. 

 The booli is also known as Scotia illustrata. 



