Noetling on the Pelecypods. — Ruedeinann. 35 
of the hinge line as constant. NoetHng holds that it would 
seem a priori possible that the valves or the hinge line should 
have, in different genera a different position in regard to the 
principal axes of the animal. He has eliminated the miscon- 
ception arising from the assumption of the stable position of 
the hinge line by considering the oro-anal axis of the animal 
as a constant line. But even this new conception would not 
furnish the clew to the law of torsion as long as the hinge line 
is considered (as by Jackson) one radius of the angle of tor- 
sion ; with Noetling's data a comparison of the valves of Nu- 
cula with any sinupalliate pelecypod valve shows, that though 
in both the axis of the hinge is parallel to the oro-anal axis, a 
torison of 90° of the valve of Nucula has taken place with ref- 
erence to that of the sinupalliates. 
The suggested law of torsion, however, can not be clearly 
conceived without a thorough understanding of the relations of 
hight and length of the shell. The investigation of this re- 
lation led to the cognition of the fact that the term "hight," 
as conventionally derived by placing the shell in a vertical po- 
sition with the ligament forming the dorsal line, is an in- 
definite term, in many cases is not homologous and in 
Pecten and Venus may represent very different axes. Noet- 
ling, therefore, defines "hight" and "len.-^th" by reference to 
the oro-anal axis, viz. the length of the shell as the dis- 
tance between two points nearest the oral and anal extremities 
and which are hence situated upon the oro-anal axis. With 
this dimension derived from the position of the animal within 
the shell the empirical "length" coincides, in the Dimyarians, 
but not in the Heteromyarians. In a shell of Pecten or Avic- 
ula we customarily measure the hight upon the oro-anal axis 
(See AO in diagram) and the so-called length upon a line 
connecting the dorsal and ventral margins. The terms have 
hence been simply inverted and it is evident that confusion 
herein cannot be avoided. 
In the Dimyaria (see Loxogoniacea of diagram) the length 
goes through both adductor muscles, in the Heteromyaria 
(Opisthogoniacea of diagram) the length passes between the 
two and in the Monomyaria (Symptogoniacca of diagram) 
the muscle has been so far removed from the oro-anal axis that 
a line, passing through the center of the former is perpen- 
dicular to the latter. 
