MR. S. O. RIDLEY ON THE GENUS DIRRIiOPALUM. 477 



The name, under the form Plocamium, was long ago applied 

 to a genus of seaweeds by J. P. Lamouroux (' Besume de Phyto- 

 graphie,' vol. i. 1828, p. 38), and adopted by Kiitzing and subse- 

 quent writers. This genus has therefore precedence of Plocatnia ; 

 and as a change is obviously necessary, I shall adopt throughout 

 the rest of this paper the name Dirr hop alum*, which Prof. P. 

 M. Duncan has suggested for the genus distinguished by Schmidt. 



Two definitions have been given of the genus, the one by 

 Schmidt (I. c. supra), the other (ostensibly a definition of the new 

 group to which he assigns it, but practically, as being the 

 only genus included, a generic diagnosis) by Sollas (Ann. & 

 Mag. Nat. Hist. (5) iv. p. 47). Both need modification in the 

 present state of our knowledge. Thus Schmidt, assuming, on 

 insufficient grounds, that the second of his two species would 

 prove to be upright and branched in the adult state, attributed 

 this character to the genus, " Schwiimme mit incrustirender Basis 

 und darauf sich erhebendem astigem Geflecht," which must give 

 way, considering that no branching specimens of the species in 

 question have yet been described. Sollas's definition includes 

 the same hitherto unjustified character. An examination or 

 study of the descriptions of the different species now assigned to 

 the genus shows that the diagnosis should stand at present : — 



" Echinonematous Sponges. Growth iucrusting or upright : 

 in the former case formed by a basal lamina of a dumbbell- 

 shaped spicule characteristic of the genus, from which spring tufts 

 of acuate or slightly spinulate spicules radiating from axes formed 

 by larger smooth acuates or subspinulates, which are enclosed 

 by ceratinous or pseudo-ceratinous fibre ; when the growth is 

 upright, the spicular tufts are set in whorls on fibres which are 

 at right angles to the axis of the stem, branch, or frond, and 

 which are similarly connected by horizontal fibres containing the 

 dumbbell spicule. Plesh-spicules, if present, a tricurvate (German 

 * Bogen') or equianchorate (' Haken '), or both." 



With regard to the distinctness of the dumbbell form of 

 spicule, which has justly been made a prominent character of the 

 genus, it must be remembered that it differs but little in the 

 principles of its construction from another type (the " tibiella " 

 of Carter, " cylindrical" or " subfusiformi-cylindrical" of Bower- 

 bank) occurring in several well-known sponges, as in the genus 



* cf«s twice, and ponaXov a club, alluding to the doubly clavate or dumbbell- 

 shaped spicule. 



