Reviews — C. B. Wakotfs Fossil Medusce. 83 



mud, sandstone, or shale ; but he is very firmly convinced that these 

 silicified bodies are the casts of naedusEe, and explains their present 

 condition by supposing that, like the recent Polijclonia frondosa of 

 the West Indian seas, the organism lived on a muddy bottom, with 

 the dorsal surface of the umbrella — disk downwards and the tentacles 

 turned upwards ; and thus, under favourable conditions of burial by 

 a rapid deposition of sediment, the watery contents of the medusa 

 would be sufficiently retained so as to avoid a collapse, whilst the 

 sediment penetrated into the interior and had time to harden. In 

 the case of forms showing the canals the author considers " that 

 these and the other internal cavities of the organism were filled at 

 once, to a greater or less extent, by the soft siliceous ooze or mud. 

 As the animal matter decomposed, the ooze gradually took its place, 

 and then began the silicification of the sediment that resulted in the 

 formation of the cast of the medusa, and of the nodules by the 

 extension of the silicification into the surrounding sediment." 



These " silicified medusae " are so full}' described and illustrated 

 in the monograph that palgeontologists will be enabled to form their 

 own opinion upon them. Many of this sceptical race, particularly 

 those who have picked their way on a sandy shore strewn over with 

 the unsubstantial bodies of jelly-fishes, will probably hesitate in 

 accepting these solid forms as medusEe. The author states that he 

 has considered the alternative question whether these peculiar bodies 

 may not have been siliceous sponges, but the absence of spicules 

 in a large number of microscopic sections examined, and the fact that 

 some of the forms occur as compressed films in the shale, led him to 

 reject this view of their origin. 



In the same family with the Middle Cambrian forms, Professor 

 Walcott also places the Dactyloidites asteroides, Fitch, sp., which 

 occurs as dark stains in the slates of Lower Cambrian age in 

 Eastern New York. This fossil was originally described as a marine 

 alga, but the author regards it as the body of a medusan flattened in 

 the slate, and not the oast of the interior. 



With respect to the fossil medusae from the Lower Cambrian 

 of Sweden, we think that the impression of Medusites Lindstromi 

 figured by Dr. Nathorst in " Sveriges Geologi " and reproduced in 

 this monograph, fully establishes the existence of medusae on this 

 horizon. 



Following the description of the PalaBozoic medusfe, the author 

 refers to those from the Jurassic strata of Bavaria, and then notices 

 those reported to occur in flint nodules from the Chalk of Europe. 

 A collection of these latter from the Natural History Museum of 

 Hamburg proved on examination to belong to sponges, as had 

 indeed been suspected by Dr. Gottsche, and not improbabl}' other 

 reputed medusa in flint will be found to have a similar origin. 



The admirable series of plates which accompanies and illustrates 

 the text of Mr. Walcott's Monograph greatly facilitates the under- 

 standing of the author's views in regard to the remarkable bodies he 

 has so minutely and carefully described. G. J. H. 



