334 PLAICE xci. 



Two of tlie teeth, on one side of the spiculum, are 

 distinctly defined while the opposite two appeal' obso- 

 lete. This deceptive ajDpearance apparently arises 

 from the dentge being very thin and flat, and as the 

 spicula are rarely evenly balanced on the bow, those 

 on one side present their flat surfaces to the eye 

 while the thin edges of the other two are scarcely 

 visible. 



The bihamate spicula vary in size to a corresponding 

 degree ; the length of one of the largest being g4-,- inch 

 in length while many of the smaller ones did not 

 exceed -2170U inch in length. Among the latter de- 

 scription of retentive spicula the contort form appeared 

 to be quite as numerous as the simple bihamate 

 ones. 



The spination of the skeleton spicula varies to some 

 slight extent. The spines are strongly produced at 

 the base of all of them, but the spination of the shafts 

 and apices of the spicula are comparatively few in num- 

 ber, and frequently they are but very sparingly pro- 

 duced. 



Leuconia Somesii, Boiverbanli. 



Plate XCI. 



Sponge, in the young state, a congeries of thin 

 fistula3, like a Lcucosolcnia ; when adult, massive, fur- 

 nished with numerous thin conical or cylindrical 

 cloacal organs, very variable in size and length. Sur- 

 face of the mass smooth and even. Small cloaca3 

 furnished with numerous long, slender, acerate, ex- 

 ternal, defensive spicula projected ascendingly at small 

 angles to the surface ; large cloacae nearly destitute of 

 external defensive spicula, furnished with a few long, 

 slender, acerate, procumbent spicula; internal defen- 

 sive spicula of cloaca), spiculated, cqui-angular, tri- 

 radiate ; spicular ray slender and attenuated. Oscula 

 minute, distributed on the inner surfaces of the 

 cloaca), Pores unknown. Dermal membrane pellucid, 



