262 PALAEONTOLOGY OF ILLINOIS. 



Canal system and interior skeleton like that of A. mam- 

 mulata. 



Diameter of a large specimen 168 mm.; entire height about 

 50 mm.; night of expanded portion 37 mm.; depth of cup 28 

 mm.; thickness of wall from 10 to 18 mm. 



This species is readily distinguished from all others here des- 

 scribed, by its extremely roughened under surface. 



Position and locality: Twenty-five feet above the base of the 

 Trenton limestone, near Dixon, Illinois. 



ANTHASPIDELLA GRANDIS U. & E. 



PL II. fig, 2, 2a. 



Sponge very large, consisting of a discoidal expansion, the 

 upper surface of which is flat, or very slightly concave. Outer 

 margin sub-acute or rounded. The entire upper surface is 

 marked by frequently connected radiating furrows, the regu- 

 larity of which is more or less disturbed at very unequal inter- 

 vals by the development of an osculum. Oscula rather incon- 

 spicuous and unequal, from 12 to 50 mm. apart. Lower sur- 

 face somewhat roughened, and traversed by frequently con- 

 nected or only serrated radiating furrows. Over limited portions 

 of the surface they are sometimes almost labyrinthically inter- 

 twined, when their radial character may be entirely obliterated 

 or, at least, much obscured. 



No complete specimen of this species has as yet been found, 

 but judging from the fragments before us the sponge attained a 

 diameter of no less than 500 mm., while its entire height does 

 not appear to have been more than perhaps two or three 

 inches. The fragments show further, that, though the sponge 

 was extremely expanded, the thickness of the wall never exceeded 

 20 mm., while it is usually not more than 12 or 15 mm. 



The shape and large size of the sponge, as well as the irregular 

 disposition of the oscula, serve readily to distinguish this spe- 

 cies from any of the associated forms. 



Position and locality : Twenty-five feet above the base of the 

 Trenton limestone, near Dixon, Illinois. 



