CALCAREOUS SPONGES. 719 



distinction from Leucosolenia, but here again we know that no 

 sharp separation can be drawn between the two types of external 

 form, for one and the same colony may be reticulate in its lower, 

 and non-reticulate in its upper portion. 



In short, we do not think that any spongologist who has 

 examined a large and representative collection of Homoccelidje 

 would be prepared to maintain Professor Minchin's subdivision 

 of the group into Olathiinidse and Leucosoleniidse. It appears to 

 us that the most that can be done at present is to pick out and 

 diagnose in a more or less satisfactory way certain more or less 

 isolated and well-characterised generic forms, and to leave the 

 vast bulk of the species in the genus Leucosolenia. 



At the same time we are of opinion, as already pointed out in 

 the introduction, that Professor Minchin has indicated some 

 characters at any rate which will in the future prove to be of 

 very great value for taxonomic purposes, and we ourselves have 

 made extensive use of the position of the nucleus in the collared 

 cells, as roughly determined from the spirit material, in support 

 of our views as to the evolution of the heteroccel Calcarea. The 

 reason why this character appears to be of less value amongst the 

 Homocoelidfe will be discussed in the section dealing with the 

 phylogeny of the group. 



There can be no doubt that the Homoccelidse have all been 

 derived from a common Olytithus -like ancestor, from which a 

 number of lines of descent have branched out in various directions. 

 Colony formation seems to have played the chief part in the 

 process of evolution and many different types of colony have thus 

 arisen. 



At present we are only able to distinguish four genera in this 

 family, Leucosolenia, Ascute, Ascyssa and Dendya. Three of 

 these, Ascute, Ascyssa and Dendya, are easily and clearly definable, 

 possessing well-marked charactei'S, bvit the remaining genus, 

 Leucosolenia, is distinguished almost entirely by negative cha- 

 racters, and contains a veiy large number of species, presenting 

 a very great diversity amongst themselves, both in colony-form 

 and spiculation. 



Genus 1. Leucosolenia Bowerbank [1864-1882]. 



Diagnosis. Diverticula of the gastral cavity, if any, never radially 

 arranged around a central tube. Skeleton composed of tri- 

 radiate or quadriradiate spicules, to which oxea may be 

 added. No uteoid dermal skeleton. Nucleus of collared 

 cells basal or apical. 



For illustrations of this genus see Dendy [1891 A]. 



The external form in species of this genus ranges from simple 

 Olynthus-like individuals, which may be connected together by a 



