SPONGES. 261 



t\veen and beyond the oscula the surface is radiately furrowed. 

 The furrows are usually half the width of the intervening 

 sponge substance. They bend down toward the lower surface 

 more abruptly than usual. It is due to the last fact that when 

 but slightly worn, the sponge presents the appearance of being 

 traversed by vertical canals, the rounded apertures of which 

 are closely arranged in radiating series (see PL I, fig. 3.) The 

 canals on the lower side of the sponge wall are usually irregu- 

 larly serrated at their sides, or they may inosculate with each 

 other without losing their distinctly radial character. From 

 six to eight occur in a space 10 mm. wide. 



The internal structure conforms with that given for the genus. 



Diameter of a specimen 88 mm.; thickness of wall from 5 to 

 8 mm.; height of expanded portion of sponge 13 mm.; height 

 of stem unknown, apparently not more than 15 mm. 



The small size and great number of the oscula characterize 

 this species, and serve to distinguish it from A. scutula. and 

 A. florifera, both of which have a somewhat similar form. 



Position ;uid locality: Twenty-five feet above the base of the 

 Trenton limestone, near Dixon, Illinois. 



AXTHASPIDELLA SCfTULA U. & E. 

 PL m. fig. 1. la. 



Sponge large, saucer shaped, rather shallow, and abruptly ex- 

 panded above the short sub-cylindrical stem. Margin sharp. 

 Under surface veiy uneven, with rough, irregular prominences 

 and depressions, which are often arranged in a somewhat con- 

 centric manner. On account of the irregularity of the under 

 surface, the radial character of the furrows is less distinct than 

 usual. Upper surface concave throughout, excepting just at the 

 inside of the outer margin where it is convex. Oscula of mod- 

 erate size, numerous, from 5 to 12 mm. apart, and surrounded 

 by from eight to fifteen radial canals. On account of the great 

 number of the oscula, the radiating furrows or canals inoscu- 

 late freely with each other. This is especially the case over the 

 central region of the surface ; nearer the margin their arrange- 

 ment in series radiating from the centre of the sponge becomes 

 more apparent. 



