MOOK, ^ ARIATIOX IX SPIRIFER MUCRONATUS 179 



Table II. — -Numher of individuals and percentages of total number of 

 individuals of various indices 



Adult Neanic 



Index f ^ ^ , '^ ^ 



Number Percentage Number Percentage 



1.26-1.50 3 10.3 



1.51-1.75 11 37.9 



1.76-2.00 9 31 2 6.8 



2.01-2.25 5 17.2 5 17.2 



2.26-2.50 1 3.4 6 20.6 



2.51-2.75 10 34.4 



2.76-3.00 2 6.8 



3.01-3.25 3 10.3 



3.26-3.50 1 3.4 



Total 29 99.8 29 99.5 



The adult curve of iiialtip/icatus is iian^ow and regular. The neanic 

 c-urve is broad and very irregular. 



I -•^' .A 



/ / 







(a)-; s •-^•'^ 



i ^ I i L 



Fig. 2. — Curves shoioing comparative conditions of shell indices in adult and neanic 

 stages of Spirifer mucronatus mut. multiplicatus 



Mutation III. — Spirifer mucronatus mut. profundus Grabau ms. 



This mutation occurs with attenuatus in the upper shales in the Mich- 

 igan region. It is small with extremities not at all or only slightly 

 mucronate. The median sinus is deep and angular. It extends from 

 the beak to the anterior end, then beyond into the brachial valve. The 

 bottom of the sinus is subangular. The fold on the brachial valve is 

 shallow and has no median depression. The plications are round and 

 close together. There are 8 to 12 on each side of the fold or sinus, 



