232 PERCY SLADEN TRUST EXPEDITION 



(5) The metasters are decidedly smaller in the " Sealark " specimen and do not show 

 so strong a tendency to pass into plesiasters. 



(6) The microxea are much fewer in the "Sealark" specimen and the tendency to 

 roughening of the surface is less pronounced, but it is very slight even in the "Challenger" 

 specimen. 



(7) The plate of which the sponge is composed is more than twice as thick in the 

 " Sealark " specimen as in the type of P. tenuilaminaris, but in spite of the specific name, 

 somewhat unfortunately chosen, this character cannot be regarded as of great importance. 

 Lendenfeld [1903] regards P. tenuilaminaris SoUas as a synonym of P. crassiuscula 

 SoUas, but I doubt whether this is justifiable in the present state of our knowledge. 

 The chief distinguishing feature of the species appears to be the absence of short-spined 

 spirasters. 



Previously known Distribution. South of Japan, 775 fathoms (" Challenger"); Japan 

 (Lebwohl). 



Register No., Locality, &c. lxxii. 2, Amirante. 



Genus YoDOMiA Lebwohl [1914]. 



Pachastrellidse in which the principal megascleres are calthrops (or short-shafted 

 trisenes), mesotrisenes and oxea, with various derivatives of these often exhibiting very 

 abnormal characters. The microscleres consist of amphiasters (or possibly some other form 

 of aster) and microrhabds. 



This genus has recently been proposed by Lebwohl [1914] for the reception of a 

 remarkable Japanese species, Yodomia ijimai, which agrees with Triptolemus in the 

 presence of mesotrisenes but difiers in the possession of calthrops (or trisenes) and oxea as 

 well, thereby approximating to the more typical Pachastrellidse. The presence of abnormal- 

 looking derivatives of the megascleres, sometimes forming spheres, appears also to be very 

 characteristic. 



Lebwohl gives the following diagnosis of his new genus "Pachastrellidse mit lang- 

 schaftigen Trisenen ; mit radial orientirten Plagiotrisenen und Mesotrisenen an der Ober- 

 flache." It appears to me that he has here laid undue emphasis on the long-shafted 

 trisenes, which are far from being typical long-shafted trisenes and are said to be relatively 

 scarce. In the new species discovered by the " Sealark " expedition they do not occur 

 at all. 



The genus Triptolemus, proposed by SoUas in 1888, includes small encrusting forms, 

 and may possibly be regarded as having been derived from Yodomia by reduction of the 

 spiculation. 



Schmidt's Stelletta pathologica [1868] from the coast of Algiers, redescribed by SoUas 

 in 1888, also includes mesotrisenes in its spiculation and is perhaps a nearly related form. 



3. Yodomia perfecta n. sp. 



(Plate 44, figs. 2, 2a; Plate 45, fig. 3.) 



The external form of the sponge is irregular and variable. Thus R.N. x. 1 (Plate 44, 

 fig. 2) forms a flat, spreading crust about 8 mm. in thickness, with an uneven, nubbly 



