154 AXXALS XEW YORK ACADEMY OF SCIENCES 



The adult curve is rery narrow considering The large number of 

 indiTiduals measured. It is resrular in fomi. The neanic curve is fairlv 

 regular and is very wide. It would correspond fairly well with a flat- 

 tened-out curve of the adult. 



Thedfordense was evidently derived from alpenense along with pro- 

 fundus. It differs from profundus in having vestiges of a plication in 

 the sinus and groove in the fold, in being more robust, and in having a 

 lower shell index. Profundus has j^rogressed further in the deepening 

 of the fold and sinus^. and thedfordense further in reduction of the shell 

 index. 



MuTATiox T. — Spinfer mucronatus mut. aiienuaius Geabau :iis. 



Attenuatus occurs in the upper shales in the ^Michigan region along 

 with profundus. The shell is very wide and short, giving a! high shell 

 index. The sinus of the pedicle valve has a distinct median plication 

 which extends nearly to the beak. The fold of the brachial valve is flat 

 and level with the plications on either side or slightly elevated above 

 them. It has a pronounced median groove which extends nearly to the 

 beak. The growth lines are moderately strong. There are 15 to 18 

 strong plications on each side of the fold and sinus. 



Maximum width 4Tnim. 



Minimum Tvidth 19 



Maximum length. 17 . 5 



Minimimi length S 



Highest shell index i adult ~i 3 . 75 



Lowest shell index ( adult) 1 . 37 



Average shell index (adult) ( about "t . . 2 .25 



Highest sheU index i neanic stage ) 1.70 



Lowest shell index t neanic stage i 2 . 23 



Average shell index t neanic stage t ( about) , . 3.40 



Table V. — Xuniijer of individuals and percentages of total numter of 

 individuals of various indices 



Adult Neanic 



Index 



Xumbor Percentage Number Percentage 



1.26-1.50 2 2.7 



1.51-1.75 7 9.4 



1.76-2.00 16 21.6 



2.01-2.25 IS 24.3 1 1.3 



2.26-2.50 14 IS. 9 6 8.1 



2.51-2.75 10 13.5 7 9.4 



2.76-3.00 4 5.4 13 17.6 



3.01-3.25 9 12.1 



3.26-3.50 2 2.7 14 18.9 



