SPONGES. 255 



DESCRIPTIONS OF LOWER SILURIAN SPONGES. 



The collection described on the following pages, comprises, 

 without doubt, the most interesting and important addition to 

 our knowledge of Palaeozoic sponges ever made. The collection 

 is so varied, and the preservation of the numerous specimens 

 in ii^neral so favorable, that we have been able to make out 

 witli reasonable certainty, representatives of no less than three 

 orders and ten genera. The Lithistida include by far the great- 

 est number of the species, and the genera Hindis, Antbaspidella, 

 Zittrfella, Edriospongia, and Streptospongia, as well as a section 

 of Calathium. The minute structure of these genera, though 

 mostly of a type not well recognized before, always contains 

 the essential characters of the order. The remaining genera 

 Strotospongia. Dyataetospongia and Camerocladia, apparently 

 belong to the Calcispongiae. 



All the specimens were collected by Dr. Everett at a quarry 

 situated three miles northwest of his home in Dixon, 111. They 

 a iv found only in a shaly layer of Trenton limestone, from 

 one to four or five inches thick, lying between heavy layers of 

 sub-crystalline limestone, about twenty-five feet above the top 

 of the St. Peter's sandstone. It is called the mud layer by the 

 quarry men, but on examination it is found to be largely made 

 up of organic remains. The shale is crowded with the stems of 

 Pal&ophj-cus and Biithotrephis, while the enclosed thin slabs of 

 limestone contain many of the characteristic fossils of the 

 Trenton group. The sponges may be scattered all through the 

 layer, or confined to particular spots in which they are es- 

 pecially abundant. Some of the slabs from these favored spots 

 are literally covered with fragmentary and complete specimens. 



