350 PROF. E. A. MINCHIN ON THE liRITISU [Dec. 18, 



carry out this intention a large numbei' of specimens, including 

 the types of eai-lier authors, have been cai-efully examined. This 

 task has been foi-ced upon me as the result of some investigations 

 upon the development of the various forms of spicules in sponges 

 of the genus Lexicosolenia, which it is hoped to publish shortly. 

 It Avas found that in describing the spicule- development it would 

 l^e necessary either to use names foi- the species which were in- 

 correct or else to employ a nomenclature at Aariance with that in 

 the current literature dealing with these sponges. And since the 

 utmost confusion exists with regaixl to the designation of these 

 species both in the labels of museums and collections, no less than 

 in even the most recent works dealing with them, it seemed woi-tli 

 while trying to give a thorough description of their distinctive 

 charactei's once and for all. It was my oi-iginal intention to 

 have inchided the Mediterranean species in this memoii', but 

 lack of material forced me to confine myself for the pi-esent to 

 the British forms, which, moreover, ai-e the more impoi'tant from 

 the taxonomic point of view, as including the earliest described 

 sponges of the genus. 



The generic name Leucosolenia Bwk. is used b}' difierent authoi's 

 in difierent senses, but is employed in this memoir in the same 

 sense as in my former publications [15, 16], namely, to include 

 those Ascons which form a natural group distinguished by the 

 following charactei'S : — 



(1) The sponge-body or colony grows in a more or less erect 

 form with i-elatively large, distinct oscular tubes. (2) All tln-ee 

 kinds of spicules are jjresent — monaxon, tiiradiate, and quadri- 

 I'adiate. (3) The ti-ii-adiate systems have two paired angles, less 

 than 120°, and an unpaired angle greater than 120°, corresponding 

 to a straight median ray and two curved latei'al I'ays. (4) The 

 collar-cells have the nucleus apical in position, situated close 

 under the origin of the flagellum. (5) The larva is an amphi- 

 blastula, and the first spicules formed are monaxons. Of these 

 characters, all except (2) are family charactei'S, distinguishing the 

 LeucosolenUdce from the Clathrinichv ; the latter having a reti- 

 culate form, equiangulai' triradiate systems, Imsal luicleus in the 

 collar-cells, and parenchymula lai'va, the fii'st spicules to be formed 

 l:)eing triradiates. The presence of all three kinds of spicules 

 distinguishes Leucosolenia from Haeckel's genus .Iscy.ssa, in which 

 monaxons alone are alleged to be present^ — a genus which, if it 

 exists, should probably be placed in the family Leucosoleniida'. 



II. Historical Review axd Criticism. 



The earliest descriptions of species of Ascons Avei'e based en- 

 tirely on outward form and appearance, and though the spicules 

 were noticed and figured no use was made of difiei-ences in spicu- 

 lation in order to distinguish the species. The external characters 

 of an Ascon are not a very safe guide, as a rule, to its specific 

 identification. In some cases, however, a species has a typical 



