424 



ME. F. A. i5ATHER ON PETALOCEI^'TJS. [Aug. 1 898, 



from p. mirabilis in consequence of the mode of preservation, one 

 might perhaps hit upon an arm-fan of P. vishycemis indistinguishable 

 from one of P. mirabilis. This led me at first to regard both as a 

 single species, and as differing mainly in the manner of fossilization. 

 More careful study and more material has shown that, not only are 

 the limits of variation in the Gotland specimens much vrider than in 

 those from America, but the norm (the acme of the variation-curve, 

 the mode, as Prof. Karl Pearson calls it) is different in each set. 

 This important fact once brought out, should not be obscured by 

 confusing the two groups under one name. 



Js^o adult specimen of P. vishycensis is known to have more than 

 22 finials, whereas the t}-pe of P. inferior has '2^. This, with the 

 differing conyexity, is quite enough to distinguish the species, so far 

 a;s present material permits a judgment to be formed. 



P. vishycensis (senior). 

 (PI. XXV, figs. 12-15 : text-fig. 9.) 



Angle of arm-fan about 83° ; fan bilaterally symmetrical in shape, 



but the distal branches do not correspond in the two halves ; finials 

 24 to 28 (?) ; ventral surface gently convex, less so than in normal 

 P. vishycensis \ dorsal surface not seen ; ridges for the most part 

 wider than the grooves and fiat-topped. 



Type: Unique specimen in Eiksmuseum, Stockholm. 



Locality: Westergarn (Gotland). 



Horizon : Lower ISilurian ; upper part of bed c of Lindstrom. 



Measurements of Specimen. — Length from ventral edge of 

 facet, along middle ridge, 25 25 mm. Greatest length, which is in 

 the right (left in ventral aspect) arm-branch, 27 mm. Length of 

 side, 25 mm. "Width, from median ridge to right (left in ventral 

 aspect) side, 15'5 mm. ; this doubled would be 31 mm., but in 

 consequence of the loss of the distal edge of the left side, the actual 

 width is 29"5. TVidth of facet, about 4 mm. Depth, about 2-25 mm. 



Xumber of finials : in the left arm-branch 11, possibly 12, but 

 since the distal margin of this 



Pig. 9. — P. vishycensis (senior). 

 Tracing of arm-fan, partly 

 restored. 



arm is gone, there may well have 

 been one or two more : in the 

 right arm-branch, 13 ; but since 

 this branch is imperfect on the 

 outer edge, there may have been 

 one more. The actual total of 

 finials seen is 24, but the total in 

 the perfect arm-fan may have 

 reached 28. In any case 18 

 branches reach the level of TI Br, 

 and three or four of these may 

 well have become YII Br. 



Angle of arm-fan in proximal 

 region is nearly 83°, but in the 

 distal region is reduced to 70*^, owing to the inward curvature of 

 the sides. Angle of facet indeterminable. 



[Nat. size.] 



