| 1a |
Shell decidedly longer
than high;
umbones
usually at or close to the anterior
end (in Adula, near the end of the first quarter) |
2 |
| 1b |
Shell about as high
or higher than long;
umbones
near the middle of the dorsal margin |
13 |
| 2a |
Umbones
essentially
terminal; anterior
ends of valves acute |
3 |
| 2b |
Umbones
not quite
terminal; anterior
ends of valves rounded |
4 |
| 3a |
Valves with course
radial ribs as well as irregular growth lines; periostracum
of larger specimens often partly or completely worn off; length
frequently exceeding 10 cm; in relatively exposed situations, mostly on
the open coast |
Mytilus
(Mytilus) californianus |
| 3b |
Valves without coarse
radial ribs, and with rather regular growth lines; periostracum
usually shiny black (brown in small specimens) and generally
persistent;
length
rarely exceeding 7 cm; common in protected situations but also found on
the open coast |
Mytilus
(Mytilus) trossulus |
| 4a |
Greatest height
of shell usually only slightly greater than the height
at the level of the umbones
(except in Adula diegensis); width
sometimes greater than the height;
mostly burrowing in shale (except Adula diegensis,
which is attached
to rocks, pilings, etc.) |
5 |
| 4b |
Greatest height
of shell usually about 1.5 times the height at the level of the umbones;
width
not greater than the height;
attached to firm substrata but not boring |
8 |
| 5a |
Posterodorsal slopes of valves hairy (photo),
and generally with an accumulation of clay particles; boring in shale
or
attached to rocks, pilings, etc. |
6 |
| 5b |
Posterodorsal slopes of valves with rough, chalky
encrustations and
transverse
wrinkles, but not hairy; boring in limestone, sometimes other rocks
(not
likely to be found north of California) |
Lithophaga (Diberus) plumula |
| 6a |
Valves with filelike vertical striations; length
sometimes exceeding 6 cm; boring in shale (not likely to be found north
of Oregon) |
Adula falcata |
| 6b |
Valves smooth or with fine radiating striations on
their anterior
portions; length
not (usually) attaining 5 cm |
7 |
| 7a |
Valves generally tapering posteriorly
(the height
at the level of the umbones
is not usually exceeded behind the middle, but there are exceptions);
genrerally
boring in shale but sometimes attached to rocks, other mussels, etc. |
Adula
californiensis |
| 7b |
Valves generally higherposteriorly
than near the middle (the greatest height
is usually about 1.5 times the height
at the level of the umbones);
attached to rocks or pilings |
Adula diegensis |
| 8a |
Length
from about 2 to about 3 times the height
(if slightly less than 2 times the height,
then relatively large--up to more than 10 cm long) |
9 |
| 8b |
Length
about 1.5 to 1.75 times the height |
11 |
| 9a |
Maximum length
about 3 cm; periostracum
greenish, often with wavy, brownish markings; periostracum
not elaborated into soft bristles (introduced, along with oysters, into
estuarine situations) |
Musculista senhousia |
| 9b |
Length
commonly greater than 5 cm; periostracum
usually brown or blackish brown, periostracum
of young speciment elaborated into soft bristles |
10 |
| 10a |
Length
about twice (or slightly less than twice) the height
and width |
Modiolus
(Modiolus) modiolus |
| 10b |
Length
about 3 times the height
and width
(not likely
to be found north of California) (M. flabellatus is
here considered
to be a synonym) |
Modiolus (Modiolus) rectus |
| 11a |
|
|
| 11b |
|
|
| 12a |
|
|
| 12b |
|
|
| 13a |
|
|
| 13b |
|
|
| 14a |
|
|
| 14b |
|
|
