In a message dated 9/26/00 1:27:11 PM EST, [log in to unmask]
writes:

<< , Conch-Lers!
    I am now fairly clear about the distinction between N. nodosus and N.
fragosus.   Now, arthriticus is given as the species name for the knobless
Lion's Paw.  But a number of things are not clear to me:  (1) must the shell
be completely knobless?  (2) what is the range of this spcies; does the range
overlap the ranges of nodosus and fragosus; (3) Are there any other
distinguishing characteristics, other than the absense of
 knobs?
  >>
Dear Linda,
  N. arthriticus differs from subnodosus in rib count, spacing and the left
valve node scheme. It has more ribs, 10-11 compared to 8-9 and a tendancy
toward grouping about the central space in the right valve. The valves are
equally convex the ears are unequal. The central and lateral key ribs are
commonly nodose.
  If you have American Conchologist from 1995, I did two articles with photos
and descriptions of arthriticus and subnodosus as well as other confusing
Pectens. The article was in two parts, one in March and one in Sept.
  I hope this helps.
        Carole Marshall