182 AXXALS XEW YORK ACADEMY OF SCIEXCES 



the adult stage, but only a mere triile lower in the neanic stage. This in 

 itself is an indication of progression toward a low indexed condition. 



3IrTATiox IT. — Spirifer mucronatus :MrT. thed for dense Shimek axd 



GrEABAU 



The mutation thedfordense occurs in the upper Hamilton shales in the 

 region about Thedford, Ontario. 



This mutation is the most advanced of the five mutations studied. It 

 is fairly good-sized and very stout. There are 12 to 15 strong sharp 

 plications on each side of the fold and siaus. The sinus is deep and 

 subangular in the adult and occasionally has a faint elevation in the 

 \ery young stages. The fold is somewhat elevated and rounded in the 

 adult, and often shows a faint median depression in the young stages. 

 The growth lines are very prominent, and are sharply deflected over the 

 fold. SLQus. and plications. Two hundred and thirty-one individuals 

 were measured. 



Maximum width 45iiim. 



23 



Minimum width 



Maximum length 3.5 



Minimiim length 15 



Highest shell index (adult) 1.^ 



Lowest shell index (adult) S9 



Average shell index (adult) t about • . . 1.30 



Highest shell index (neanic staged 3.56 



Lowest shell index (neanic stage > 1 . 21 



Average shell index (neanic stage) (about i . . 2.03 



Table IV. — Xum'ber of individuals and percentages of total niiml)er of 

 indhviduals of various indices 



AduU Neanic 



Index 



-1.00 

 1.01-1.25 

 1.26-1.50 

 1 . 51-1 . To 

 1.76-2.«Xj 

 2.01-2.25 

 2.26-2.50 

 2.51-2.75 

 2.76-3.00 

 3.01-3.25 

 3.26-3.50 

 3.51-3.75 



r 



Number 



I't-rcenrajre 



Number 



Percentage 



24 



10.3 











107 



46. 



o 

 •J 



o 



.8 



80 



34. 



6 



17 



7.3 



15 



6. 



4 



54 



23.3 



5 



2. 



1 



63 



26.8 













40 



17.3 













24 



10.3 













18 



7. 7 













10 



4.3 













1 



.4 













1 



.4 













1 



.4 



Total. 231 99.7 2.31 99.0 



