Vol. 54.] MR. F. A. B.iTHER ON PETALOCRINUS. 407 



line of cleavage is not so regular. (PI. XXVI, figs. 58-65, & text- 

 figs. 11, 12, p. 432.) 



P. inferior. — U.C. 4736, in the Walker Museum, found close to 

 the locality of P, mirabilis, but at a slightly lower horizon, by 

 Mr. Weller, who kindly forwarded me the specimen. The matrix is 

 a loose dolomitized limestone, in which the calcareous fragments, 

 echinodermal and other, are not wholly dissolved out. The specimen 

 is an impression of an arm-fan, in which, ridges represent natural 

 casts of the arm-grooves. There are two fractures, and the margins 

 are worn away. (PL XXVI, fig. 57, & text-fig. 10, p. 427.) 



P. visbycensis. — The specimens are all composed of calcspar well 

 preserved in a blue shale ; they are usually dark in colour. 



Specimens in the Riksmuseum, Stockholm: — Twenty-three arm- 

 fans or portions of arm-fans, lettered a-w ; details of some are 

 given with the measurements (p. 423). (PI. XXV, figs. 1-11 & 

 16-25, & text-fig. 8, p. 423.) 



Specimens in the British Museum (Nat. Hist.) : — 



(x) E 6638 ; an arm-fan on matrix, ventral surface exposed ; it is 

 slightly worn at the distal and proximal ends (text-fig. 8 x, p. 423). 



(y) E 6639 ; small arm-fan, somewhat weathered. 



(z) E 6640 ; proximal portion of an arm-fan. 



P. visbycensis (senior). — A single arm-fan on a matrix of hard 

 calcareous shale, ventral surface exposed : Riksmuseum, Stockholm 

 (PI. XXV, figs. 12-15, & text-fig. 9, p. 424). 



P. angustus. — A single arm-fan, the proximal end broken away, 

 especially on the left side ; Riksmuseum, Stockhohu (PL XXV, figs. 

 26-32). 



P. expansus. — A single arm-fan on spathose limestone, ventral 

 surface and facet exposed ; slightly weathered ; Riksmuseum, 

 Stockholm (PL XXV, figs. 33-36, & text-figs. 13, 14, p. 435). 



' P. (?) major J — HoJotype and topotype, both belonging to 

 Mrs. Davidson, and kindly withdrawn by her from the Walker 

 Museum. 



Mr. Weller wrote to me on Nov. 10th, 1896 : — ' I am convinced 

 that the specimen I described as P. (?) major is nothing but a form 

 of coral. Additional specimens secured this summer seem to make 

 this certain/ Examination of these specimens suggests that the 

 species is most closely allied to Omjphyma turbinatum (Linn.). 



IV. Anatomical Description of the Genus. 



The cup, stem, and first primibrach are known only in P. mira- 

 bilis, but there is no reason to suppose that they were essentially 

 different in the other species. The arm-fans of all species are 

 known, and it is on them that specific diagnoses must be based ; 

 their structure in P. mirabilis and P. visbycensis, which difi'er but 

 slightly, may be taken as the norm of the genus ^; they are more 



^ On the zoological appHcation of the word norm, see F. A. Bather, 

 ' Postscript on the Terminology of Types,' Science, n. s. vol. v, pp. 843-44, 

 May 28th, 1897. 



