Noetling on the Pelecypods. — Rnedeinann. 37 
less than the length. For these the term Loxogoniacea is pro- 
posed. By far the majority of all pelecypods belong to this 
group, specially the large group of the sinupalliate Dimyaria. 
3) a r= 90°, tan a =z I. Hight and length are nearly 
equal. 
In this case axis of growth and the oro-anal axis form a 
right angle and the growth takes place perpendicular to the 
oro-anal axis and in ventral direction. Pectunculus is mention- 
ed as the best example for this group, the Orthogoniacea. 
4) a > 90°, but < 180°, tan a < I. but with different 
sign than case 2. 
The two axes form an obtuse angle and growth takes place 
in antero-ventral direction ; Prosogoniacca. These correspond 
to the Loxogoniacea (see diagram) and a right valve of the 
Loxogoniacea is similar to a left valve of the Prosogoniacca. 
Mesodesma donacina and Nucula are mentioned as character- 
istic representatives and it is stated that in depending solely 
upon the external characters of the shell, one would consider 
the longer anterior part as the posterior one, as is done indeed 
in most descriptions. 
5) a irr 180° ; tan « = o ; H = o. 
Auipliigoniacea. These correspond to the Symptogoniacea, 
both axes coincide again, but growth takes place in opposite 
direction, viz. forward. No examples are known to the author, 
but it is thought that they would have to be looked for among 
paleozoic forms, Avhich might have the pallial sinus or the' 
posterior muscle near the umbo. 
6) a > 180° ; but < 270°; tan <>. y o, but < i ; H < L. 
Here the Schalenschiefe forms an angle in the third quad- 
rant and mathematically we have the case of the Loxogoniacea ; 
and upon the shell the two axes would form an acute angle, 
but in fact the angle is greater than 2 R (R = right angle, 
3ee diagram), and grow^th takes place in dorso-oral direction, 
i.e. the ventral organs would lie loelow the umbo and the dorsal 
organs near the free shell margin. No examples of these, the 
Protoconchac, are known, and the author concedes that it will 
be difficult to find any; but adds that among such paleozoic 
Dimyaria as have an acute ''Schalenschiefe" a i)allial sinus near 
the umbo should be sought. 
7) a = 270°; tau'/ — 1 ; H = R. 
