1916 Du Bois; on Brachiopods 181 



considered in measuring the variation. The length of the shell divided 

 by the width was taken as the index of the shell. The measurements were 

 made with a vernier caliper^' (plate 31) and are accurate to .05 mm. The 

 angle considered is the angle made by the valves at the anterior margin. 

 The measurements were made with a contact goniometer and are accurate 

 to .5 degree. The average of all the indices of the individuals of a habitat 

 was taken as the index of the habitat." A glance at the figures in table 



Table 2. Showing indices and anterior angles of shells from various 



depths. 

 Habitat No. Shells IndexAver. Angle 



No. 1. Turn Island 80 .824 84 



No. 2. Cattle Point 5 .853 81 



No. 3. Brown Island 16 .868 78 



No. 4. 35 fathoms 45 .883 77 



No. 5. 50 fathoms 95 .887 77 



No. 6. 80-90 fathoms 53 .895 70 



No. 7. 10-90-20 fathoms 837 .884 77 



/unprotected 1 .840 85 



No. 8. Turn RockJ ( . . 1 .865 78 



{;;; 



(protected \.. 1 .870 77 



2 will show that there is a definite tendency toward the development of 

 the shorter, more gibbous shells in the habitats where the animals are 

 exposed to rough water. The same result is produced by the strong wave 

 action at the surface and by the action of strong currents below the sur- 

 face. This result is to be expected when the manner of growth of the 

 brachiopod is considered. Professor H. W. Shimer, in his work entitled 

 "An Introduction to the Study of Fossils," says concerning their growth: 

 "The soft body of the animal, lying at the posterior portion of the 

 shell, occupies only about one-third of the interior. The body wall gives 

 off two folds or mantles, one fitting closely to and secreting the pedicle 



valve, the other secreting the brachial valve Any marked 



injury to a mantle is necessarily reflected in the shell. If something in- 

 jures a mantle edge, the first process in healing is a puckering up of 



the mantle around the injured place which causes a like 



puckered appearance in the shell at that place; as the mantle becomes 

 nealed the growth lines of the shell become more and more regularly 



5This caliper was designed by Dr. Slielford after a suggestion by Dr. 

 Chas. B. Davenport, of Cold Springs Harbor, Long Island, N. Y., and was 

 provided by tlie Zoology Department of the University of Illinois. 



6The writer is indebted to Dr. R. D. Carmichael, of the Department of 

 Mathematics, University of Illinois, for suggesting this method of determin- 

 ing the index of the habitat. 



