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



and the point is sometimes so reduced in dimensions as to suggest 

 that it might be readily lost altogether ; in one instance it was 

 found replaced by a blunted, but almost smooth extremity ; thus 

 the only serious difference between these species lies in the 

 character of the pointed end of this spicule. It seems to me that 

 we have here the very point of transition from Clathria to Dir- 

 rhopalum, and for these reasons I believe in a close affinity between 

 the two genera. And this fact is the more interesting, as Prof. 

 Schmidt has called attention to the British Sponge-fauna as con- 

 sisting of an aggregation of indistinctly differentiated forms. 



Sollas (Ann. & Mag. N. H. (5) iv. p. 49) found gradations be- 

 tween the dumbbell spicule of D. plenum and the spined and basally 

 inflated acuate of the skeleton. May his transitional forms not 

 show rather that the dumbbell spicule of the secondary fibre was 

 originally like that of D. JBeani, a spined spinulate or acuate, 

 which is now only represented by these occasional reversions to 

 the primitive type ? 



Existence of Dirrhopalum in the Fossil State. 



This fact appears to be indicated with some probability by the 

 figure given by Mr. Carter (Ann. & Mag. N. H. (4) vii. p. 133, 

 pi. ix. fig. 50) of a spicule from the Upper Greensand of Haldon 

 Hill, near Exeter, which corresponds in size to the average dimen- 

 sions of the dumbbell spicules of Dirrhopalum. It has a smooth 

 shaft, and smooth large extremities sharply distinguished from 

 the shaft. 



Prof. Sollas (op. cit. (5) vi. p. 392, pi. xx. fig. 46) figures and 

 describes, as the basis of a provisional new genus and species 

 called Rhopaloconus tuberculatus, a large subcorneal spicule 

 rounded at each end, and covered with stout tubercles, just such 

 as those of the two distinctive spicules of D. clopetarium, Schmidt. 

 Its size, however, is 95 by *24 millim. It may perhaps represent 

 an ancient divergence from the spined acuate form in the direction 

 of a simple cylinder. 



A. K. Zittel, in his memoir on the genus Coeloptychium (Abh. 

 math.-phys. Kl. bayer. Akad. Wiss. xii. pt. iii. p. 1), figures, 

 among a large number of spicules obtained from fossil Sponges 

 of that genus, some (viz. pi. iv. figs. 20, 51, 65) which seem likely 

 to have belonged to species of Dirrhopalum of the clopetarmm 

 section ; they belong to the Upper Cbalk. His fig. 17, a very 

 remarkable form, with slight smooth shaft and large strongly 



