216 PALAEONTOLOGY OF ILLINOIS. 



portance as factors of the palaeozoic fauna will soon manifest 

 itself. Thousands of specimens are now relegated to the "trash 

 boxes" of collectors, nearly all of whom have something among 

 their undetermined fossils, that would add to the list. Again, I 

 suspect very much, that many of the so-called "Fucoids" really 

 represent the remains of sponges, as has already been shown of 

 Dictyophyton Hall, (Hydnoceras Conrad,) Uphantsenia Van- 

 uxem, and Cyathophycus Walcott. 



THE TACONIC SYSTEM, according to Walcott*, already repre- 

 sents a sponge fauna comprising six genera and thirteen species. 

 The figures of Leptomitus zitteli Walcott, (loc. cit.) suggest 

 Monactinellid affinities, or, what is perhaps more likely, they 

 represent basalia of some Hexactinellid sponge. The spicules of 

 Protospongia have all the essential characters of the Hexacti- 

 nellidse. The peculiar genera, Archseocyathus Billings, and 

 Ethmophyllum, Meek, are referred to the class with much doubt. 

 In certain respects they suggest a line of evolution through 

 Beatrices into the Stromatoporoids, w r hile other peculiarities 

 remind us of the Zoantharia Rugosa. 



CAMBRIAN SYSTEM.! Sponges are somewhat disproportionately 

 distributed in Cambrian strata, but this is in part due to the 

 unequal care exercised by collectors in searching for them. Thus 

 the Trenton and Cincinnati groups have for many years been 

 subjected to the most energetic searchings, and from these strata 

 we have a correspondingly large number of sponges. None are 

 as yet known from the Potsdam, but from the Calciferous group 

 five species are described by Billings, belonging to the genera 

 Calathium ?, Ethmophyllum, Rhabdaria and Trichospongia. 

 Billings also describes from the uncertain Quebec group, five 

 other species of Calathium, and the genus Trachyum, with two 

 species. 



* C. D. Walcott, 2d Contr. to the Cambrian Faunas of N. A. Bull. U. S. Geol. Surv., 

 No. 30. 



t The term Cambrian as here used is equiyalent to the Lower Silurian of Mur- 

 chison and of the majority of American geologists; of the Ordovician of Lapworth, and 

 of the Cambro- Silurian of the Canadian geologists. This arrangement seems to me to be 

 the only just and practical settlement of the Taconic- Cambrian controversy which is re- 

 ceiving so much attention from geologists at present. 



