282 PALAEONTOLOGY OF ILLINOIS. 



STOMATOP0110IDS. 



ACTINOSTROMA? TRKNTONENSE U. & E. 

 PL VII. Fig. 3, 3a, 3b. 



The specimen which we propose to designate as above, is a 

 rounded sub-hemispheric mass, 47 mm. in diameter, by 23 mm. 

 in height. It consists of superimposed layers, from 1 to 3 mm. 

 thick. 



Sections are somewhat obscure, but show thai? the mass is 

 composed of minute irregular vertical rods, of stellate form in 

 cross section. At varying intervals the rods throw out from 

 four to six slender rays which may or may not unite with the 

 rays of the surrounding rods. 



In its minute structure this fossil resembles the ANTHASPIDELLID.E, 

 but the fact that the hemispheric mass is made up of superim- 

 posed layers, seems to indicate a more decided affinity with the 

 Stromatoporoid section Actinostroma,, proposed by Dr. Nichol- 

 son. The typical species of Actinostroma,, one of which is illus- 

 trated on plate VII, differ from our species in having the rays 

 of the "radial pillars" placed at corresponding levels. Up to 

 the present time the genus w r as not known to have representa- 

 tives in older rocks than those of the Wenlock limestone of 

 Britain. From that horizon two species are mentioned by Dr. 

 Nicholson. In Devonian deposits the species are more numerous. 



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

 Trenton limestone, near Dixon, 111. 



