General Classijicallon of the Pehcypoda. 



109 



Those ribs would then be developed which were of greatest 

 importance to the shell — they might be those nearest to the 

 hiiiffc-line or those furthest from it. 



The second apparent objection is the alteration of a ventral 

 snccession in the Taxodonta and some Dysodonta to a dorsal 

 one in the remaining Dysodonta and all the Heterodonta if, 

 as seems probable, they have a common origin. This objec- 

 tion has already been partially answered, where it was stated 

 that those ribs which were most important would develop 

 first. The change in order of development, then, may merely 

 mean a change in the relative importance of the upper and 

 lower ribs. This may reasonably be accounted for on the 



Fio-. 15. 



LPI 



LPI 



Left. Right. 



Chdtnn l(ixarii.«:. (After Beruard.) 



firmly established principle that those parts of an organism 

 most highly developed in the adult tend to appear first in 

 ontogeny. In the early shells, the Palseoconchs, the shells 

 were thin and would be likely to break under the strain of 

 the ligament. The most external, that is the uppermost ribs, 

 which are more than mere valleys between the external ribs, 

 and, moreover, need not be associated with external ribs, 

 would be extremely likely to be useful as helping to 

 strengthen that part of the shell. Being more developed 

 they would appear sooner than the less important ventral 

 ones. After the appearance of the cardinal plateau, or even 

 after the general thickening of the shell, this use would be 

 subordinated to the use of guiding the shell to ensure rapid 

 and accurate closing. This would be better accomplished 

 by ventral ribs, which would then develop first. 



Noettling reasons from diagrams 12, numbers 2 and 3 (see 

 text-fig. 10), of Bernard's work on Heterodonts, that lamella 

 III, which is shown as curved round, is more differentiated 



