92 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 9i 



Forest. Then, in the summer of 1939, Drs. Josiah Bridge and G. A. 

 Cooper had the opportunity of studying the area and obtaining addi- 

 tional collections besides confirming Dr. Kirk's location. They report 

 that outcrops in the canyon itself afford good collections of the 

 sponges particularly on the north side about two-fifths of a mile 

 inside the entrance. Here the best fossils are found on a talus slope 

 50 to 70 feet above the valley floor below the big cliff, although some 

 may be collected from outcrops in the several ravines. 



Associated with those sponges and the trilobite Pliomerops is an 

 undescribed fauna of Ostracoda, a few stony Bryozoa, crinoid and 

 cystid remains, trilobites, cephalopods, gastropods, and brachiopods. 

 Of the last, the follow^ing species were described as new by Ulrich 

 and Cooper in 1936 ^ : Aporthophyla typa, Toquimia kirki, Goniotretiia 

 peiylexa^ Rhysostrophia nevadensisj, and R. occideritalis. This part 

 of the Pogonip limestone seems to be represented elsewhere in North 

 America in the Table Head formation of Newfoundland and the Oil 

 Creek formation of Oklahoma. 



These Nevada fossil sponges are preserved in a thin-bedded, dense, 

 clayey limestone composed largely of organic remains and often 

 weathered enougli at the surface to show silicification of the contained 

 fossils. With further etching by acid the minute spicular structure 

 of the sponges can be seen to better advantage at their surface, but 

 farther within where water has not penetrated the spicules have the 

 same calcareous structure as the rest of the matei-inl. In practically 

 all jDublications on the order Tetractinellida of the Silicispongiae. 

 authors describe the spicules as originally siliceous but explain that 

 when found calcareous the silica has been replaced by lime. Should 

 that be true, all these early as w^ell as later Paleozoic sponges have 

 without exception been so replaced, a phenomenon that certainly has 

 not occurred so uniforml3\ These sponges undoubtedly follow the 

 rule of all other Paleozoic fossils that whenever they are buried in a 

 calcareous siliceous shale or certain clayey limestones the organic 

 calcite is replaced at the surface by silica, but the original structure 

 on the interior remains calcium carbonate just as it does in most other 

 fossils. Associated with these sponges are great numbers of long, 

 needle-shaped structures, which may be dermal spicules. These are 

 here illustrated (pi. 21, fig. 7) as a doubtful species of Hyalostelia^ 

 but their relationship, if any, to the associated sponges has not been 

 discovered. 



The original abbreviated descriptions of the following species, with 

 the exception of one new form, appeared in the Journal of the 

 Washington Academy of Sciences, volume 17, No. 15, pages 391-394, 



^Journ. Pal., vol. 10, pp. 616-631, 1936. 



