HEREDITY. 411 



EXPLANATION OF FIG. ng. 



LETTERING. 



a. Apex of shell. This usually bears a scar on the point, as shown in 

 Nos. 14 and 15, but this has no bearing on the question discussed, and has 

 not been described. This also represents the youngest (nepionic) or cyrto- 

 ceran stage in the growth of the shell, No. 8 being a young shell with complete 

 living chamber. This letter also indicates the location of the sections corre- 

 spondingly lettered in the figures. 



b is used to indicate the section of the cyrtoceran stage in Nos. 11-13. 



b' is used to indicate the place of the sections, Nos. 4-5^', upon the whorls 

 of Nos. 4-5. They were taken through the whorl in the gyroceran stage. 



cis used for the adolescent (neanic) stage of growth in the whorl and the 

 corresponding sections. 



c' is used for the full-grown (ephebic) stage in the growth of the whorl and 

 the corresponding sections. 



d for the first part of the senile (gerontic) stage. 



efor the final and most degenerative part of the senile stage. 



iz for the impressed zone. 



v venter or outer side of the shell, the dorsum being the inner side of the 

 whorl. 



w for the whorls, thus i w in Nos. 3 and 4 means the end of the first whorl, 

 2 TO the beginning of the second whorl, 3 w that of the third whorl. These 

 letters serve to show the progressive increase in numbers of the whorls in the 

 different classes of forms. 



FIGURES. 



No. i. Outline of an orthoceran shell. 



No. 2. Outline of cyrtoceran shell. 



No. 3. Outline of gyroceran shell. 



No. 4. Outline of nautilian shell, having a larger umbilical perforation 

 at (a) and fewer whorls at the same age, than in No. 5 ; in other words, it is 

 less tightly and completely coiled up than the class of shells represented by 

 that figure. 



No. 5. A nautilian shell with tighter coils than in No. 4 and the whorls 

 coming in contact and the impressed zone beginning at an earlier stage. 



No. 6. Barrandeoceras bohemicum (sp. Barrande) Hyatt, showing the most 

 involute of the Silurian shells so far as known; No. 6 is reduced in size, but 

 the section No. 7 is natural size. 



No. 8. A young shell of the same, natural size, with complete living 

 chamber. 



Nos. 9-10. Coloceras globatum (sp. De Koninck) Hyatt, adult. No. 9 has 

 a part of the outer shell broken off, showing the edges of the septal partitions 

 (sutures) as lines on the strong cast of the interior. 



Nos. 11-13. Same to show the cyrtoceran stage and section, with its im- 

 pressed zone. 



No. 14. Cenoceras clausum, Hyatt. 



Nos. 15-16. Nautilus pompiiius, to show the cyrtoceran stage with its im- 

 pressed zone. 



