A MONOGRAPH OF AVEST AMERICAN PYRAMIDELLID 



MOLLUSKS. 



By William Healey Dall and Paul Bartsch, 



0/ the Division of Mollusks, U. S. National Museum. 



INTRODUCTION. 



The Pyramidellidse are a family of mollusks mostly of small size 

 and world-wide distribution. No record of unmistakable Pyrami- 

 dellidge has been found in the Cretaceous faunas, though such may 

 occur, but they are numerous in the Tertiaries and perhaps most 

 fully represented in the existing faunas. 



Numerous names have been applied to them; sometimes a particu- 

 lar group has been supposed to have no pHcation on the pillar, a mis- 

 take which can be corrected by grinding down the whorls so as to 

 expose the interior more fully than may be done by inspecting the 

 natural aperture. The plication in such cases is present, but falls 

 short of reaching a point where it can be observed through the 

 aperture. 



In all the species of which the soft parts are known the external 

 anatomy is very similar. In examining a large number of species, as 

 noted by Fischer, intermediate types occur, until it becomes a matter 

 of great difficulty to decide where, if anywhere, the generic lines can 

 be drawn. It is not surprising that some authors have resorted to 

 the expedient of regarding most of the species, notwithstanding the 

 contrasted extremes of the series, as belonging to a single genus. 

 When a group is composed of such a multitude of species, it seems 

 more convenient in practice and leads more efficiently to clear 

 thinking to take the other view, and subdivide the groups sufficiently 

 to make it reasonably clear where a given species belongs in the 

 series. 



In the absence of anatomical characters it has been necessary to 

 fall back in large part on the form and sculpture of the shell the 

 presence or absence of an umbilicus, and the character of the plica- 

 tions on the pillar, as distinctive characters, although it must be 

 admitted that between the different sections some intermediate forms 

 may occur. So many names have been applied to members of the 



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