82 Revieics—C.D. Walcotfs Fossil Medusce. 



be present at all, it is inconsiderable in amount. No concentric 

 structure is to be seen in tbe nodules, but some sbow lamination 

 marked by parallel mica scales, as if the nodule were produced by 

 a silicification of a portion of the stratified mud or shale. A chemical 

 analysis of a portion of a nodule within the body of a medusa showed 



83 33 per cent, of silica, nearly 4 per cent, of calcite, and 4 per cent, 

 of alumina; whilst the shale in which the nodules are imbedded 

 cmsists of 55 per cent, of silica, 21 per cent, alumina, and under 

 2 per cent, calcite. 



The bodies described as silicified medusse are very varied in form ; 

 so much is this the case that it is difScult to find any two alike. 

 Those placed under the genus Broolcsella are for the most part 

 discoidal or cake-shaped bodies from 1 to 5 ctn. in diameter, with 

 a depressed convex upper surface, consisting of a number of lobate 

 arras or rays extending from a common rounded central area. These 

 lobes are considered to be the umbrella of the jelly-fish ; there are 

 from six to more than twenty of them in different individuals; soine 

 are narrow, subcylindrical, and terminate obtusely, whilst others are 

 expanded or club-shaped distally. On the under surface of these 

 bodies thei'e is another series of arms, proceeding, like those of the 

 upper surface, from the central area or axis, which are regarded as 

 the lobes of the sub-umbrella; and, further, in some specimens, 

 between the upper and under series, there is yet another set of 

 lobes, attached to the central axis, belonging to the inter-umbrella. 

 This peculiar combination of lobate arms " results in a very complex 

 structure, and instead of a simple symmetrical medusa, an irregularly 

 lobed body results, with round appendage-like lobes projecting out 

 in various directions from the central disk or axis." In each lobate 

 arm of these diiferent series a simple radial canal extended, and 

 opened into the central axis which is believed to represent the central 

 stomach of the organism. Judging from the figures, these radial canals 

 are often shown as small circular openings at the distal extremity 

 of the lobes, or, when exposed in section, as narrow infilled tubes 

 of a different coloration in their axial line, and they are very 

 similar in appearance to the canals of fossil sponges when infilled 

 with silica. No trace of an oral opening has been noticed in these 

 bodies, and in only one specimen an oral plate, formed by the union 

 of four arms at the centre, has been recognized. Tentacles are 

 unknown. 



In a second genus, Laotira, there is a still more complex develop- 

 ment of lobate arras, and in some compound forms three radiate 

 centres appear to be present, and the lobate arms from each inter- 

 communicate with one another. 



Assuming that these peculiar bodies are fossil medusse, the 

 question naturally arises as to the mode in which they can have 

 been preserved, so that not only the numerous lobate arms and 

 appendages, but even the axial canals within them, retain their 

 original form as solid casts in silica. The author fully recognizes 

 that, according to present opinion, such delicate organisms can only 

 have been preserved as fossils by faint impressions in fine calcareous 



