450 Prof. W. J. Sollas on the 



region outwards are more numerous than in the smaller ; and 

 in the largest a branch from the axis downwards seems to be 

 a second rootlet. The spinispirules do not differ from those of 

 the adult ; but the quadriradiate spirules are absent from the 

 two youngest forms. 



The slender spicules of the anchoring-fibres, over which 

 the ectoderm extends, are mostly rounded at the distal end 

 (PI. XVII. fig. 39), like many of the spicules oi R. schosnus, 

 or the forms which so frequently occur as varieties amongst 

 the pin-shaped acuates. These represent the first stage of the 

 grapnel-spicules, which thus differ from the similar spicules 

 in Tetilla by the absence of an initiatory inflation. In the 

 next stage (PL XVII. figs. 33-38) these spicules exhibit 

 near the distal end a number of little tubercular excrescences, 

 similar to those which occur as abnormal thickenings on 

 many of the spicules both of the Monaxonidaj and the Tetrac- 

 tinellidse. In many cases these tubercles take the form of 

 small teeth, often recurved, and varying in number from one 

 to six. They are seldom situated at the extreme end of the 

 spicule, usually a little distance from it. In the larger 

 specimens we find a considerable advance in growth and de- 

 velopment ; the spicules show a marked increase in size ; and 

 though some of these larger forms still present a merely rounded 

 end, others possess in addition from one to three short conical 

 teeth budded off at some little distance before theend (PI. XVII. 

 figs. 40 to 42). There is still not the slightest trace of any 

 terminal inflation, such as occurs in Je/«7/a-grapnels. The rays 

 arise merely as spines, precisely similar at this stage to the 

 more numerous spines which cover the distal end of the quadri- 

 radiate spicules of Tricentrium muricatum. We may indeed, 

 on the basis of these observations, regard the rays of these 

 grapnels as highly developed spines^ which, at their inception 

 indefinite in number, become subsequently limited to three. 

 The club-shaped spicules, pi-eviously mentioned as the pro- 

 bable parents of the forks, have also been observed in these 

 young forms ; but no spines have yet been found proceeding 

 from them. The bifurcated forks, however, are in these early 

 stages very small, their rays being 0*006 inch long, while 

 those of the adult are 0*05 inch, or eight times as long. 



Class i/j cation. — Thenea is evidently a true tetractinellid 

 sponge ; but it differs from those hitherto described in this Re- 

 port by the complete absence of a cortex, and thus is a typical 

 example of our Leptochrota ; this character has been noticed 

 long ago by Sir Wyville Thomson, who, in his paper onHoltenia^ 

 recognized its classificatory value, and founded his suborder 

 Leptophlcea upon it. This suborder is nearly the same as my 



