Vol. 54.] MR. F. A. BATHER ON PETALOCRINUS. 439 



YII. SlTMMART. 



This paper discusses the Silurian Crinoid genus Petalocrinus, 

 Weller & Davidson, 1896, on the evidence of all the original 

 material from Iowa, and of further material from Iowa, Indiana, 

 and Gotland. 



Petalocrinus is shown to have a dicyclic base, not monocyclic, as 

 originally described. The structure of the tegmen is shown to be 

 that of the Cyathocrinoidea. The arm-fans, characteristic of the 

 genus, are proved to have been formed by fusion of the branches of 

 an arm of Cyathocrinid type. In them are described for the first 

 time axial canals, covering-plates, the articular facet, and various 

 minor structures. The species P. (?) major^ Weller, is shown to be 

 an Omphyma ; but P. mirabilis, Weller, the genotype, is redescribed, 

 and with it five new species — P. inferior and P. longus from Iowa ; 

 P. vishycensis, P. angustus, and P. expansus (as well as a possible 

 mutation, P. vishycenms senior) from Gotland. The family Petalo- 

 crinidae, at present including only this genus, is diagnosed, and 

 arguments are offered for its descent from Cyathocrinidse, probably 

 by way of AraclmocHnus. 



EXPLANATION OF PLATES XXV & XXVI. 



[All figures are twice the natural size (that is, X 2 diameters), except 

 where the contrary is stated.] 



Plate XXV. 



Specimens from Gotland, drawn by Gustaf Wenman. 



Fetalocrimis vishycensis (p. 421). 



Fig. 1. (g) Ventral view; shows scalloping of edge, and perfect ridges. 



2. (g) Dorsal view ; shows depressions corresponding to ventral ridges, 



3. (g) Distal end, ventral surface upwards ; shows indentation of ridges. 



4. (f ) Ventral view ; shows perfect ridges with notched edges. 



5. (e) Type. Distal end, ventral surface upwards. {Cf. fig. 3 & text- 



fig. 8, p. 423.) 



6. (e) ,, Ventral view ; shows nearly perfect ridges with notched 



edges. 



7. (e) „ Dorsal view; shows shagreen ornament, growth-lines, 



scalloped distal margin, and a Beyrichia embedded in 

 secondary stereom. 



8. (ll) Ventral view ; shows grooves opening in places into the axial canal. 



X 5 diam. 



9. (p) Ventral view ; the axial canals for the most part confluent with the 



grooves. X 5 diam. 



10. (p) Proximal end of fragment ; showing grooves I, TI, III, IV, as 



explained on p. 419. X 5 diam. 



11. (n) Proximal end ground down; for interpretation, see text-fig. 6 on 



p. 418. X 6 diam. 



12. (senior) Elevation, as seen from distal end, showing transverse curva- 



ture. (6y. fig. 33.) 



13. ( „ ) Ventral view. (C/". text-fig. 9, p. 424.) 



14. ( „ ) Articular facet, left bottom portion broken. 



15. ( ,) ) Section along line of fracture a-h\ shows traces of original 



sutures in middle of ridges. X 5 diam. 



