478 MR. S. O. RIDLEY ON THE GENUS DIRRHOPALUM. 



Alebion, Gray ( = Halichondria Pattersoni, nigricans, pulchella, 

 Bowerbank, &c), and Tedania, Gray (T. nigrescens, Muggiana, 

 digitata, Schmidt, &c), Cribrella hospitalis, Schmidt, and fore- 

 shadowed in the long cylindrical of Suberotelites mercator, Sdt., 

 and Desmacidon columella, Bowerbank, in which a magnifying- 

 power of about 400 diameters reveals a slight inflation. The 

 " tibiella " also occurs in Hymeniacidon armatura, Bowerbank, 

 Suberites fuliginosus and in Halichondria infrequens, Carter, and 

 Desmacidon diance, emphysema, physa, anceps, Schmidt. As to its 

 relations to other linear spicules, see p. 485, where the systematic 

 position of the genus is discussed. 



The dumbbell spicule may, however, be distinguished from the 

 "tibiella" by its having the maximum length not exceeding 20 of 

 its own maximum diameters, and by its being always arcuately 

 curved. 



I propose to give notes on the specie3 to be assigned to the 

 genus, adding, in the case of those which are now assigned to it 

 for the first time, the reasons which have led me to adopt this 

 course in their respective cases. Beginning with Schmidt's own 

 species, those on which the genus was based, I find it necessary 

 to supplement his short descriptions by fuller details, taken from 

 the microscopic preparations supplied by himself to the British 

 Museum. It will be seen that the result of an examination of 

 these preparations justifies, in part, Sollas's supposition (Ann. & 

 Mag. 1ST. H. (5) iv. p. 4G) that Schmidt had perhaps overlooked the 

 flesh-spicules of his species. 



1. DlRRHOPALTJM GYMNAZON. (Plate XXIX. figs. 1, 2.) 



Plocamia gymnazusa, Schmidt, Spong. atl. Geb. p. 62, pi. iv. fig. 17. 



To the details given by Schmidt (Z. c.) should be added the 

 following : — 



Bases of echinatiug- and main-fibre spicules connected by 

 yellow ceratinous (?) * material. No spinulate spicules as stated 

 by Schmidt. 



Skeleton-spicules of three kinds : — (1) Long, slender, smooth, 

 slightly curved acuate, tapering from the base to the sharp 

 point ; size 2*13 to 2*48 by '04434 millim. (2) Stout smooth 

 acuate, tapering to a sharp point from within quarter of its length 

 from the base, and slightly tapering to the base from the same 



* As the slide appears to have a talc cover, the test of polarization (see 

 p. 481 ad init. and note) is inapplicable here. 



