NAPLES FAUNA IN WESTERN NEW YORK, PART 2 .2.2.5 



For the interpretation of this phenomenon we may be guided by the 

 remarkable observations recently published by Noetling on the morphology 

 of the lamellibranchs, specially that on "Das Torsions Gesetz der Schale." 1 



By careful determination of the oral and anal extremities in living 

 species, an oro-anal (o-a) axis is located which for purposes of reference is 

 regarded as a fixed or datum line. The line of greatest growth {Crescenz- 

 linie, w and w in figure 7) or that along which the shell increases 

 most rapidly, is variable for different groups and makes in the series 

 actual and hypothetic (as illustrated in Noetling's diagram), all possible 

 angles with the oro-anal axis. As the beak is always the initial extremity 

 of the crescence line and the latter is variable, the former also changes 

 position with reference to the datum. 



Noetling has shown the inaccuracy of the current orientation of the 

 shell with reference to the animal and hence of the descriptive terms 

 "hight," "length," "ventral," "dorsal," "anterior" and "posterior" as 

 usually applied, and which circumstances still compel us to adopt. For 

 mathematical reasons, based on the angle between the oro-anal and cres- 

 cence axes, Noetling erects eight ordinal groups departing in two directions 

 from an elementary condition, hypothetic and believed to be paleozoic, 

 which he designates " Protoconchae." 



There may of course be all possible intervals between these groups 

 which represent only positions on the circle. From the Protoconchae 

 departure is in one direction toward the dimyarians through the Amphigoni- 

 acea (no living representative ; to be sought in the Paleozoic), the Proso- 

 goniacea (Mesodesma, Nucula), the Orthogoniacea (Pectunculus), the 

 Loxogoniacea (the majority of sinupalliates). In the other direction from 



'These papers are the following: Beitrage zur Morphologie des Pelecypodenschlosses. 

 Neues Jahrbuch fur Mineral. Beilbnd 13. 1900. p. 140; Notes on the Morphology of the 

 Pelecypoda. Paleontologia Indica, new series 1. Mem. 2. 1899; and that specially 

 referred to here, Beitrage zur Morphologie der Pelecypoden. Neues Jahrbuch fiir 

 Mineral. Beilbnd 15. 1902. p. 394. An English abstract of the last with plate has been 

 given by Ruedemann in Am. Geol. Jan. 1903, p. 34. 



