84 Mr. J. S. Bowerbank on Moss Agates 



apice erosa; anfractibus quatuor, apertura cserulescente, efFusa. 



PI. VI. fig. 8. 



Hab. Rio Sacramento, California. 



Distinguished from P. nuclea of Mr. Isaac Lea, which is 

 from a neighbouring locality, by its somewhat smaller size, 

 bluish instead of white mouth, having one whorl less, the 

 aperture more expanded, and being without the black line 

 round the mouth, which, when present, is so good a character 

 in his shell, but which, in my numerous specimens of it, I do 

 not find at all constant, and usually only to be seen in those 

 better developed. 



August 1, 1842. 



XIV. — On the spongeous origin of Moss Agates and other sili- 

 ceous bodies. By J. S. Bowerbank, Esq., F.G.S. 



[Concluded from p. 18.] 



In the green jaspers the organic structure of the tissue is 

 often preserved in the most extraordinary manner. The whole 

 of the sponges that are found in this substance that I have 

 examined are referable to that division, which I have proposed, 

 in the paper " On the structure of the keratose sponges of 

 commerce/' to designate Fistularia, from the fibre being fur- 

 nished with a central cavity like that seen in Spongia fistula- 

 ris of Lamarck. In one case, especially, which is represented 

 by PL II. fig. 5, the dimensions of the fibre and of its cen- 

 tral tubes, the size of the interstices, of the network and its 

 mode of arrangement, are, as far as can be ascertained from 

 the small specimen in which they are imbedded, so exactly 

 similar to those of Spongiafistularis, PI. II. fig. 6, as to render 

 it exceedingly difficult to believe them not to be the remains 

 of the identical species in a fossilized state. In the paper on 

 the keratose sponges of commerce read before the Microsco- 

 pical Society*, I have described one species of the Turkey 

 sponges, and some of the Australian ones as having their 

 solid fibres surrounded by a horny sheath, in which a system 

 of minute anastomosing vessels were imbedded ; and as be- 

 fore stated, we find in Spongia fistularis the fibre furnished 

 with a continuous central cavity ; but I could not detect in 

 either of the two specimens of this sponge that I have had 

 the opportunity of examining any traces of a vascular sheath 

 on the external surface of the fibre. The existence of the 

 combination of these two interesting forms of structure in the 



* Trans. Microscopical Society of London, vol. i. p. 37. pi. 3. figs. 11, 12 

 and 13. 



