Vol. 54.] MR. P. A. BATHER ON PETALOCRINUS. 425 



In the proximal region the ridges are no thicker than in average 

 specimens of P. visbycensis^ and the arms branch neither more nor 

 less. Towards the distal margin branching is less rapid, and this 

 is correlated with the less angle of the fan ; but since the arm does 

 continue to increase in width, the ridges widen, and in some cases 

 attain a breadth of 1'6 mm. 



The characters in which this specimen differs from normal 

 specimens of P. vishycensis all appear to be those of old age, namely, 

 increase of grooves distally, and more particularly the irregularity 

 of the increase combined with lessened angle of arm-fan and 

 widening of ridges, also greater flatuess of ventral surface. All 

 these characters, except the number of the finials, are opposed to 

 those of P. inferior, which there is no reason to regard as anything 

 but a normal adult. Therefore, though the present specimen could 

 not be included in P. vishycensis without disturbing the diagnosis, 

 it seems better not to separate it as a distinct species, but to regard 

 it as gerontic. This raises another question : Is it simply gerontic 

 or phylogerontic ? In other words : Is it a senile individual of the 

 ordinarj'^ P. visbycensis-type, or is it a mutation that has acquired 

 senile characters by acceleration of development? The large size 

 of the specimen favours the former view ; the fact that it comes 

 from a higher horizon is not incompatible with the latter. Should 

 further specimens be obtained from the same horizon and all found 

 with these gerontic characters, then the form must be regarded as 

 a phylogerontic mutation. For the present it may be distinguished 

 merely by the addition of ' senior.' 



P. angustus, n. sp. 

 (PI. XXY, figs. 26-32.) 



Angle of arm-fan 38° ; fan bilaterally symmetrical in shape and 

 number of grooves, but the levels of the branchings differ greatly in 

 the two halves; finials 10; ventral surface of fan very slightly 

 convex, except at distal margin ; dorsal surface slightly concave 

 along middle Jine, convex towards sides, with clear growth-lines and 

 shagreen ornament; ridges much narrower than grooves, with 

 crenulate tops. 



Type : Unique specimen in Riksmuseum, Stockholm. 



Locality : Shore north of Wisby (Gotland). 



Horizon : Lower Silurian, bed b or c of Lindstrom. 



Measurements of Specimen. — The proximal end is broken 

 away, especially on the left side (right in ventral aspect). Length 

 of median ridge preserved, 15'25mm.; length of right side, 15'75mm. ; 

 probable length from ventral edge of facet to distal margin, 16*5 mm. 

 Greatest width of fan, 9 mm., occurs at 3'5 mm. from the middle of 

 the distal margin ; width at margin, about 8*6 mm. Width, depth, 

 and angle of facet unknown. 



Number of finials 10 ; and the distal narrowing of the fan shows 



