Mr. H. J. Carter on new Sponges. 433 



or infinitely rapid. We could not see either in the Amoeba 

 were it not for the magnifying-power of the microscope, nor 

 in the heavenly bodies, were it not for their great size and 

 great distance. Hence we cannot comprehend this infinity, 

 and should only speak of these phenomena as they appear to 

 our finite organs of sense, modifying our assertions by our 

 equally finite reason, in all philosophic humbleness. I have 

 been induced to make these remarks because I have lately 

 observed a tendency to speak more decidedly in microscopic 

 inquiries than our powers justify. 



EXPLANATION OF THE PLATES. 

 Plate XX. 



Fit/. 1. Tethya antarctica, n. sp. ; fully developed young one, natural size. 



Fig. 2. The same, lateral view, magniKed to the scale of l-48th to 

 l-1800th of an inch : a, summit ; h, base ; c c c, vents ; ddd, pa- 

 pillpe of surface supporting spicules ; e, tufts of spicules project- 

 ing from the base ; /, two very long anchor-headed spicules 

 projecting fi'om the side. 



N.B. The greater part of the anchor-heads have been broken 

 off. 



Fig. 3. The same, end view of summit, magnified to the same scale, 

 showing three large vents, which branch off internally into the 

 excretory canals : aa a, vents ; hbh, papillae of the surface sup- 

 porting spicules. 



Fig. 4. The same, full-grown specimen ; portion of dermal sarcode, show- 

 ing the pores and spicules of the surface : a a, pores ; h, spicules. 

 Scale l-48th to l-1800th of an inch. 



Fig. 5. The same, form of acerate spicule. 



Fig. 6. The same, form of triforked spicule ; one prong much larger than 

 the other two, which are equal. 



Fig. 7. The same, form of anchor-headed spicule of the body. 



Fig. 8. The same, form of anchor-headed spicule of the tufts at the base. 

 N.B. All these are adult forms, drawn to the scale of l-24th 

 to l-1800th of an inch. 



Fig. 9. The same, real average length of largest acerate spicule. 



Fig. 10. The same, real length of longest portion of shaft, to which the 

 anchor-head remained attached. 



Plate XXL 



Fig. 1. Rossella antarctica., nov. gen. ; large cruciform peripheral spicule, 

 showing : — a a a a, the four arms, covered respectively with a 

 layer of large and small (macro- and micro-) spines ; 6, con- 

 tiiiuation of adjoining arm ; c, shaft or vertical arm, covered with 

 a layer of microspines only ; d, continuation of same, to show 

 form of free extremity. Scale. l-48th to l-1800th of an inch. 



N.B. In this figure the arms are truncated, to meet the size of 

 the plate, and drawn straight instead of sigmoid, for con- 

 venience. See the natural form in figs. 5 & 6. 



Fig. 2. The same, lateral view, to show the position of the arms relatively 

 to that of the shaft. All truncated to make the figure smaller. 



