174 



SHIMER AND GRABAU — HAMILTON GROUP OP THEDFORD 



2.9, though rising as high as 3.2 and falling as low as 2.6. The average 

 proportional width of 9 individuals of S. mucronatus var. arkonensis was 

 found to be 3.9. though rising as high as 4.5 and falling as low as 3.5. 

 It thus appears that the average neanic S. mucronatus thedfordensis is 

 more extended proportionally than adult S. mucronatus, plainly ap- 

 proaching the adult of S. mucronatus arkonensis. The adults of the three 

 varieties stand as follows : S. mucronatus thedfordensis, 1 : 1.85 ; S. mucro- 

 natus, 1 : 2.9, and S. mucronatus arkonensis, 1 : 3.9. 



The following table shows the actual width and height, proportional 

 width, and number of plications in each of three principal stages of 

 development in Spirifer mucronatus var. thedfordensis S. and G. : 





Ephebic. 



1 



Neanic. 



Nepionic. 









rZ 



m 







xi 



to 







__-. 



GO 









•+3 



fl 







+3 



a 









q 



Sh 







T3 



.2 







V, 



o 







'S 



.2 



CD 







'£ 



o3 







£ 



o3 







% 



03 



B 



3 







+i 



p 







, -^> 



o 







, -+-S 



o 







M 



'Hh 









"ft 







°§ 



'ft 



fl 







d'53 



«H 







C 53 



Vh 







s ® 



««H 



<-H 







0^5 



o 







o^ 



O 







OrG 



O 



53 



a 



rg 







CD 



rd 



-*£ 





53 



rg 



■*» 



'-t 3 O 



Pi 

 a) 



"3 



cu 

 ft 





&J0 



"53 



w 



ft 



o 



s 



-§ 

 % 



"53 



w 



ft 

 o 



u 



a 



■73 



'53 



w 



ft 

 O 



1 



1 



39. 



20.5 



1.9 



13 



34.6 



- 9.7 



3.57 



13 



8. 



4.5 



1.77 



6 



2 



41. 



20. 



2.05 



12 



37.5 



9.2 



4.07 



11 



8.8 



3. 



2.90 



8 



3 



37. 



20.5 



1.80 



14 



34. 



10. 



3.40 



12 



7.8 



3.8 



2.05 



8 



4 



37.5 



19.2 



1.95 



14 



33.3 



9.7 



3.43 



10 



4.1 



2.7 



1.51 . 



6 



5 



31. 



18.2 



1.70 



13 



28. 



9.8 



2.86 



12 



10. 



5. 



2. 



8 



6 



40. 



18.8 



2.13 



12 



38. 



9.1 



4.17 



9 



10.2 



4.4 



2.30 



6 



7. 



35.8 



16.8 



2.13 



12 



34. 



10. 



3.40 



11 



11.8 



6.7 



1.76 



7 



8 



38.5 



20. 



1.93 



14 



34. 



9.6 



3.54 



10 



8. 



4.1 



1.95 



6 



9 



32.2 



165 



1.84 



12 



28.6 



6.6 



4.33 



9 



8.3 



4.5 



1.85 



7 



10 



38. 



16.2 



2.35 



14 



35.6 



8.9 



4. 



11 



7.4 



3.3 



2.24 



8 



11 



22.3 



14.8 



1.50 



11 



30. 



8.2 



3.66 



10 



5. 



2.9 



1.72 



4 



12 



39.2 



19.6 



2. 



12 



35.7 



11. 



3.25 



9 



9. 



4.6 



1.95 



6 



13 



32.4 



18.9 



1.71 



9 



29.2 



8.3 



3.57 



9 



6. 



3.6 



1.60 



5 



14 



35.2 



21.2 



1.70 



11 



33. 



9.5 



3.47 



7 



8. 



5.6 



1.40 



6 



15 



28. 



is! 2 



1.54 



12 



26. 



8.9 



2.92 



9 



8.4 



4.6 



1.83 



6 



16 



46. 



18.3 



2.51 



18 



42.4 



11. 



3.85 



14 



7-4 



3.7 



2. 



6 



17 



39. 



18.1 



2.15 



13 



36.4 



9.8 



3.71 



13 



4. 



2.4 



1.66 



5 



18 



30.2 



18. 



1.69 



11 



36.4 



7.4 



4.90 



10 



7. 



3.5 



2. 



6 



19 



22.8 



18.8 



1.21 



9 



20.8 



10.6 



1.96 



9 



7.2 



4.4 



1.63 



8 



20 



34.4 



18.8 



1.83 



11 



30. 



8.5 



3.53 



11 



8.4 



4. 



2.10 



8 



21 



26.4 



17.6 



1.50 



11 



22.4 



6.2 



3.61 



9 



9. 



4.2 



2.14 



7 



22 



28. 



20. 



1.40 



9 



24.2 



7.2 



3.36 



10 



6.6 



3.2 



2.06 



5 



23 



40. 



21.4 



1.9 



11 



36.6 



10.5 



3.5 



8 



5.2 



3.1 



1.7 



5 



24 



43. 



21.3 



2. 



15 



39.2 



10.9 



3.6 



12 



7.4 



3.7 



2. 



5 



25 



38. 



22.4 



1.70 



12 



35.6 



9. 



3.9 



9 



5.8 



3.6 



1.6 



5 



26 



38. 



18.4 



2.06 



11 



35. 



8.1 



4.3 



11 



5.2 



2.9 



1.8 



6 



27 



42. 



20. 



2. 



12 



39. 



11.5 



3.4 



11 



8. 



4. 



2. 



7 



28 



36.2 



19.7 



1.80 



12 



34. 



11.8 



2.9 



11 



11.2 



5.5 



2. 



8 



29 



33. 



20. 



1.60 



12 



28.2 



8. | 



3.5 



8 



8.2 



4.3 



1.9 



6 



