INTRODUCTION. 



great extent omitted. This list approximately represents our present 

 knowledge, both in its acquirements and its defects, and is intended 

 as a help toward something better and not in any sense as a finality in 

 nomenclature or distribution. 



We may now proceed to an explanation of the form and scope of the 

 Tables. 



Taking the columns serially, the first carries a serial number useful 

 for check-list and exchange purposes. Then follows the name and 

 authority. Then comes a column referring to the number of the plate 

 or plates, and another for the numbers of the figures. As the figures 

 on most of the plates are drawn to very different scales, a column is 

 inserted, giving the maximum length ; axial in Gastropods, antero-pos- 

 terior in Pelecypods, of the specimen in millimeters. One millimeter 

 is practically one twenty fifth, or four one-hundred ths, of an inch, so 

 that for those unaccustomed to the metric system there is little diffi- 

 culty in reducing the millimeters to fractions of an inch. 



When no dimension is given in the column it will be understood that 

 the figure, if any, is of the size of nature ; or that its magnification or 

 diminution is stated on the plate itself, or represented there by a line 

 or other conventional sign. 



The next column states the range in depth as far as known of each 

 species in the form of a fraction, the least depth forming the numerator 

 and the greatest observed depth the denominator. Where a zero occurs 

 it indicates that the species is found at low- water mark. The maximum 

 and minimum are selected from the whole range, domestic or exotic, 

 recorded for the species in question. When no depth is stated it will 

 be understood that the species is supposed to inhabit the shallow water 

 near shore or between tides. 



This is succeeded by a column in which the extreme northern limit, 

 locality, or region of the species referred to is recorded. When this 

 relates to a locality within our special region there will seem sometimes 

 to be a discrepancy ; as, for instance, when a species appears as present 

 in the " Hatteras " column, while in the "northern extreme" column 

 Charleston, S. C., will be found. But, as will be immediately shown, 

 Hatteras in the heading of the column does not mean a locality but 

 a district, extending from Savannah, Georgia, to Cape Hatteras, North 

 Carolina, so that the discrepancy is only apparent. In the off-shore 

 dredgiugs it has been practicable sometimes to give only the latitude, 

 or a general term such as u Arctic seas," to indicate the northernmost 

 distribution of a species, since there has been no adjacent landmark to 

 cite for northern limit. When a species has its northern limit on the 

 rich archibenthal grounds off Block Island and the Vineyard, or Nan- 

 tucket, I have indicated this by " Ehode Island" in the column, since 

 this sufficiently guides foreign students who might be puzzled by the 

 other names so much less apt to be found on small-scale maps of our 

 eastern coast. The data for such species will be found chiefly in the 



