Vol. 54.] MR. F. A. BATHER 0?f PETALOCRINUS. 405 



With the disappearance of all the carbonate of lime, water did not 

 cease to penetrate the rock, but continued to deposit silica, partly as 

 quartz, filling up all the minute spaces in the stereom, lining the 

 qavities, and cementing the fragments, partly as chalcedonic mam- 

 millations on the inner walls of the hollow boxes, thus obscuring 

 details of structure that might otherwise have been preserved with 

 surprising clearness. The original replacing silica, the crystalline 

 infilling, and the chalcedony are all shown in text-fig. 2, p. 404. 



The larger fossils, therefore, are neither casts nor impressions, but 

 when perfect resemble externally the original skeleton in everything 

 except their chemical and physical composition, and consequent soapy 

 feel. Often, however, this exterior is roughened by deposit of silica 

 or marked with the concentric rings of beekite formation. But the 

 fossils, being but thin and hollow cases, empty shows, rarely are 

 perfect, and we see their interior. The eifect of this is often some- 

 what that of an impression, and from it one may take a wax squeeze 

 simulating the characters of the original outer shape. Text-fig. 1 

 (p. 401) shows the outer casing partly preserved on the dorsal side 

 of the arm-fans numbered iv & v, while it is almost entirely removed 

 from arm-fan iii. The exterior of the basals is also preserved, but one 

 sees the interior of most of the radials, especially in radius i. The 

 diverging ridges seen inside the arm-fans represent grooves on the 

 ventral surface, and this is also the case with PI. XXYI, figs. 44 & 46. 



The need for this detailed explanation of the mode of petrifaction 

 was proved by the difficulty of interpreting Mr. Weller's description 

 and photographic process-engravings without such explanation. 

 A similar mode of preservation was described in my paper on some 

 Eugeniacrini from the ' Weisser Jura e ' of Streitberg (Quart. Journ. 

 Geol. Soc. vol. xlv, 1889, pp. 359-362). 



The specimens studied are the following : — 



Specimens which Mrs. Davidson has deposited on loan in the 

 Walker Museum, University of Chicago, but which she kindly with- 

 drew temporarily for me to examine at leisure, namely : 



(a) Cotype.^ Weller & Davidson, op. cit. pi. vi, fig. 2. Crown, 



and proximal columnal, dorsal view. (Pi. XXYJ, figs. 

 41-43.) 



(b) Cotype. Weller & Davidson, op, cit. pi. vi, fig. 5. Single 



arm-fan in matrix, ventral view ; part of dorsal surface 

 and facet exposed. 

 (e) Portions of five arms, in place around a cup which is not 

 exposed ; dorsal view, but the dorsal wall and inner sub- 

 stance of the arm-plates are removed ; numerous column- 

 fragments and isolated column als, possibly belonging to 

 the species, are close by. (PI. XXYI, figs. 48-50.) 



^ In this paper the word ' type ' and its compounds are used in the senses 

 proposed by Oldfield Thomas (' Suggestions for the more definite Use of the 

 Word Type, etc.,' Proc. Zool. Soc. 1893, pp. 241, 242) and elaborated by 

 C. Schuchert ('What is a Type, etc.,' Science, n. s. vol. v, pp. 636-640, 

 April 23rd, 1897). 



