THE 



GEOLOGICAL MAGAZINE, 



NEW SERIES. DECADE III. VOL. III. 



No. IV.— APRIL, 1886. 



OS.IGtHsT-A.Ij ARTICLES. 



I. — On Some Fossil Ostracoda from Colorado. 



By Prof. T. Rupert Jones, F.R.S., etc. 



(PLATE IV.) 1 



TK December last Dr. C. A. White, Palseontologist to the U.S. 

 Geological Survey, sent me a small packet of siliceous Ostracoda, 

 obtained by dissolving in dilute acid some pieces of an impure lime- 

 stone from the Jurassic "Atlantosaurus Beds " near Canon City, 

 Colorado. These strata are of freshwater origin ; and, besides the 

 wonderful Dinosaurian and Mammalian fauna, which Prof. 0. C. 

 Marsh has published, they contain, Dr. White informs me, Unio, 

 Limncea, Planorbis, and Valvata, all of modern types. In the impure 

 limestone with Gstracods, and in associated layers, these Gasteropods 

 are found also in a silicified state, and the Dinosaurian fossils occur 

 both above and below them. 



In January of this year I received also from the U.S. Geological 

 Survey, through Dr. A. C. White's kind agency, a piece of the hard 

 limestone itself, for the examination of the microzoa not treated with 

 acid. Some fragments, carefully crushed, yielded free specimens, 

 but not so many nor so perfect as those obtained by acid-solution of 

 similar pieces. By careful manipulation my friend Mr. C. D. Sherborn 

 succeeded in finding some that showed traces of ornament by the 

 presence of the whitish matrix (siliceous) in rows of little pits and 

 in slight furrows similar to those on the Purbeck Metacypris Forbesii. 

 Consequently these little siliceous organisms are not all internal casts, 

 but some are perfect carapaces. 



On the examination of thin sections and decalcified slices, many 

 sections of Ostracodous bivalved carapaces are recognized, also 

 crushed valves and numerous fragments. The test is often apparent, 

 replaced by silica. No limbs nor internal organs are visible within 

 the whole (bivalve) tests ; only groups of brown granules, arranged 

 somewhat concentrically, as also in the neighbouring siliceous in- 

 fillings of Limncea, and other Gasteropods, and in the chalcedonic 

 (agate-like) infillings of rifts and other spaces in the matrix. In 

 some instances the carapace-valves are absent, and the first inside 

 layer of chalcedony imitates the shell. On being decalcified, a slice 



1 This Plate was drawn with aid of a grant from the Royal Society for the illus- 

 tration of fossil Ostracoda. 



DECADE III. VOL. HI. — NO. IV. 10 



