12 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



the above critical investigation, thus leaving previous more or less incorrect state- 

 ments out of account, and therefore the same notations as above are used. The 

 following survey is consequently composed of the schemes given for each group, the 

 first part representing my own idea of correspondence, the second that of Munier- 

 Chalmas: 



»Inverse» forms »Normal» forms 



cardinals laterals cardinals laterals 



I Odhner 



( left valve : x r X 



{ right valve : YA Yp y 



tt »t f fre© valve: 



II Muniek- I 



l 

 L 



Chalmas) .. , , v . v , 



[ tixed valve: 1A lp y\ l 



4 b 6b LT II 



3 a 1 3 b 5 b LP I 



3a 1 3 1) 5b bl' I 



4 b C bl LP II 



I have intentionally used the letters x and y to designate the teeth in Echino- 

 chama and the allied forms, because these were the unknown elements to which 

 homologa should be sought in the normal shells. It novv remains to discuss accord- 

 ing to which of these schemes the homologisation is to be performed. 



Comparing the »inverse» and the »normal» forms according to part I of the 

 above table, we find that there are three cardinal tooth elements in the left valves 

 of both forms. In the »normal» Chamas a division of 2 into 2 a and 2 b is trace- 

 able through the occurrence of 1 in the right valve, received beneath its middle. In 

 the »inverse» forms either such a division must exist as well, or it has been com- 

 pletely absent from the beginning. In the first case we have to recognize as tooth 1 

 either YA or Yp, in the second case YA must be equivalent to 3 a. Lastly, 

 Bernard has found as an homologon of 1 in Chama a small thickening on the hinge 

 plate margin beneath the foremost callous tooth in Echinochama. 



Which of these alternatives corresponds to reality is, of course, a question that 

 can be solved only by means of ontogenetical researches. In the next chapter we 

 will try to show the development of the hinge types in order to get a possibility 

 stating the homologies between them. 



2. The ontogenetical development of the hinge types in Chamidae. 



The very confused state of our knowledge of Chamidae has had its natural 

 cause in the enormous variability of their exteriör morphology, a feature common 

 to all sessile mollusca. The hinge exhibits, it is true, more constant characteristics, 

 but as it grows and increases in solidity many primary details change, so that in 

 old shells the hinge construction is not so sharply marked as in young ones. In young 

 shells we will find the most primitive characteristics, not only specific ones but also 

 generic ones. An investigation of the early stages of the shell is therefore of great 

 importance as a means of ascertaining the changes that take place during the de- 

 velopment of the hinge, its real homologies and consequently the relations of the 

 Chamidae to other Lamellibranchia. 



