W. E. Alkins — Morphogenesis of Brachiopoda 



TABLE II. 



Comparison of. Depth with Length. 



Length 



Depth 



No. 



Mean 

 Depth 



2 



Length 



Depth 



No. 



Mean 

 Depth 



Length 



18 



Depth 



No. 



Mean 

 Depth 



Q 



o 



4 



2 



1 



12 



7 

 8 

 9 

 10 



2 



34 



28 

 3 



8-48 

 9 20 



12 

 13 

 14 

 15 

 16 

 17 

 18 



13 

 14 

 15 

 16 

 17 

 18 



3 

 22 



19 

 9 

 2 



1 



3 



16 

 15 



4 



1 



13 80 

 14-62 



2 

 3 



1 



1 



2-88 



13 



8 

 9 

 10 

 11 



7 



47 

 18 

 2 



5 



3 



4 



15 

 9 



3 38 



19 



6 



4 

 5 



27 

 5 



4-16 



14 



15 

 16 



8 



9 



10 



11 



12 



10 

 11 



12 

 13 



1 

 9 

 32 



28 

 2 



7 



48 

 11 



4 



5 

 16 

 33 

 11 



10-29 



11-17 

 11 77 



7 



4 

 5 

 6 



7 



29 

 2 



4 87 



20 



14 

 15 

 16 

 17 

 18 

 19 

 20 



6 



12 



13 



9 



2 



1 



15-84 



8 



5 



6 



7 



25 

 21 

 4 



5-58 



9 



6 



7 



33 



18 



6-35 



10 

 11 

 12 

 13 



21 

 22 



15 

 16 



17 



18 



6 

 7 

 13 



8 



16 68 



10 



6 



7 

 8 



10 

 49 

 5 



6-92 



17 



10 

 11 

 12 

 13 

 14 

 15 



1 

 2 



27 

 26 

 14 

 3 



^2-81 



11 



6 



7 

 8 

 9 

 10 



1 

 22 



31 

 5 

 1 



7-72 



16 

 17 



18 



3 

 6 

 10 



17-37 



23 



18 

 19 

 20 



1 

 2 



19-0 

















24 



19 



1 



19 1 



The solid figures show that Reticularia lineata presents a very 

 good example of a truly homogeneous species : the variation 

 shown is perfectly continuous throughout, and there is not the 

 slightest evidence of any tendency towards a differentiation into 

 two or more groups. Such solid figures may be taken as typical 

 of " good," simple, homogeneous species. 



Ontogeny. We may now safely proceed with a mathematical 

 investigation into the ontogeny of the shell. 



From the data necessary for the construction of the length- 

 width and length-depth distribution figures the mean value of 

 the width and depth respectively corresponding to each length 

 was calculated. These mean figures are included in Tables I 

 and II. They represent the average width and depth of shells 



