186 ANNALS NEW YORK ACADEMY OF SCIENCES 



Adult Xeanic 



Index ^ '^ V '■ V 



Number Percentage Number Percentage 



3.51-3.75 1 1.3 10 13.5 



3.70-4.00 5 6.7 



4.01^.25 2 2.7 



4.26-4.50 5 6.7 



4.51-4.75 2 2.7 



Total 74 99.8 74 99.7 



The adult curve of atteiiuatus is broad and rather irregular. The 

 neanic curve is very broad and irregular. 



This mutation is in many respects the most primitive of the five muta- 

 tions studied, although it occurs in the highest of the three horizons. It 

 is nearest to muItipHcaius in form and general characters though inidti- 

 plicatus has a lower shell index. 



The derivation of atienuatus is uncertain, though it is not far removed 

 from multi plica t us. Several possibilities may be considered. Attenuatus 

 may be derived from some form of multiplicatus whose index is much 

 higher than any of those measured. It may have come from a multi- 

 plicatus stock having an index like the highest of those measured and 

 then degenerated into a short and broad form. It may have spmng 

 from a common ancestor with multiplicatus in a lower horizon and re- 

 mained primitive while multiplicatus progressed toward a lower index. 

 The third possibility is by far the most probable. The adult curve of 

 attenuatus corresponds roughly with the neanic curve of multiplicatus, 



CONCLUSIOX 



Two distinct lines of development have been observed in Spirifer 

 mucronatus. In one of these lines development has advanced to a con- 

 siderable distance from the primitive Spirifer mucronatus condition. 

 This line includes alpenense, profundus, and tkedf or dense. The other 

 line has progressed much more slowly. This line contains muUiplicatus 

 and attenuatus. In the first line thedfordense and profundus have prob- 

 ably been derived directly from alpenense. In the second line attenuatus 

 was probably not derived directly from multiplicatus, but from a form 

 closely resembling multiplicatus, but more primitive. Multiplicatus and 

 alpenense were probably derived from some primitive form of Spirifer 

 mucronatus belonging to a lower horizon than any of the five mutations 

 studied. The approximate relations of the five mutations to each other 

 are shown graphically in the following diagram. 



