Ross,
 
The basic rule is simple, but the application of it is not always.
In Latin, nouns are assigned "gender", but not on the same basis as
in english.  In english names of inanimate objects are neuter gender.
 The only masculine and feminine nouns we have are those which refer
specifically to a male or female person or animal.  Mother, sister,
hen, and ewe are feminine nouns.  Brother, uncle, stag and bull are
masculine.  However, we don't generally think about noun genders
because english adjectives do not vary with the gender of the noun
being modified.
In Latin the situation is very different.  Many names of inanimate
objects are classified as feminine or masculine nouns.  For example,
the Latin words for farmer, table, road, and water are all feminine
nouns.  Feminine Latin nouns typically end in the letter "a"
(Littorina, Cypraea, Mercenaria, Crepidula).  Masculine nouns
typically end in "us" (Conus, Nassarius, Mytilus, Fusinus).  Neuter
nouns end in "um" (Sinum, Gloripallium, Umbraculum, Buccinum).  The
rule governing Latin adjectives is that the adjective ending must
reflect the gender of the noun it modifies.  Therefore Buccinum
undatum is correct.  Buccinum undata or Buccinum undatus are
incorrect because of the gender incontinuity between the genus (noun)
name and the specific name (adjective).  Nerita undata is correct
however, for the same reasons.
Abra alba, but Nassarius albus
Sinum maculatum, Amoria maculata, Trochus maculatus
Simple!  But not always.  Some Latin nouns, especially masculine
ones, have alternative endings.
In those cases, the adjective still has to match, but sometimes the
adjective gets the alternative ending, while other times it follows
the a-um-us system.  From this point on, it gets pretty complicated.
But you can see that if a species is reassigned to a different genus,
and the new genus has a different gender, then the specific name
ending has to be changed to match the new genus.
 
Paul M.