TOKSION OF THE LAMELLIBRANCH SHELL 



An Illustration of Moetling's Law 



BY JOHN M. CLARKE 



Noetling's law of torsion in the lamellibranchiate shell, as set 

 forth in one of his recent publications on the morphology of these 

 mollusks 1 , involves the conception that the so called beak of the 

 shell is an unstable position; it is a point of inception of shell 

 growth, but in the various groups of these mollusks the relation 

 of its position to the true axis of the animal itself varies through 

 the entire circle from 0° to 360°. If thus the beak is a variable 

 in position, the current orientation of the' shell is reduced to a 

 pure convention and we must, as Noetling concludes, have ref- 

 erence to the fundamental axes of the animal, directly or by im- 

 plication, for the correct posit of the organism. Variable with 

 the beak is the line of most rapid shell growth which departs 

 from the beak, the ridge which in oblique or elongate shells is 

 commonly called the " umbonal ridge " and which for all shells 

 Noetling terms the crescence-line (Crescemz-linie) . 



The fundamental and un variable axial line is that running 

 from mouth to anus and is termed the oro-anal axis. The or- 

 dinary value of the terms " hight " and " length " as applied to 

 the shell when referred to this constant axis are lost and Noet- 

 ling regards as length the actual length measured on the oro-anal 

 axis. With this line derived from the position of the animal 

 within the shell the empirical " length " sometimes coincides in 

 forms like Venus and other Sinupalliates. In Pecten however 

 the oro-anal axis or true length, is the distance as usually ex- 

 pressed between the hinge line and the basal margin of the 

 valve, hence the " hight." Thus there is here and elsewhere an 

 inversion of current terminology. 



The angle made by the constant or oro-anal axis and the var- 

 iable or crescence-line is an index to the degree of torsion or revo- 

 lution of the animal with reference to its valves. This angle is 

 termed the Schalenschiefe or angle of obliquity (a) and of course 



Noetling, Fritz. Beitrage zur Morphologie der Pelecypoden : Neues Jahrb. 

 fur Mineral. 15 Beil. bnd. 1902. English abstract by R. Ruedemann in 

 Am. Geol. Jan. 1903, p. 34. 



