Key:  Family Lyonsiidae

Taken primarily from  Kozloff, 1987, 1996 p. 287 (Copyright 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
 

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1a	Valves generally nearly circular in outline, but not consistently the same shape, and often obviously deformed; embedded in colonies of compound ascidians, with the siphons exposed	Mytilimeria nuttalli
1b	Valves decidedly longer than high, not nearly circular in outline (often deformed, however); not embedded in colonies of compound ascidians, but may be embedded in sponges or nestling in burrows made by other animals	2
2a	Shell not often appearing to be deformed; valves with delicate ribs (these are not obvious in portions where the periostracum has persisted); pearly internally and externally, except where there is periostracum Lyonsia bracteata (Gould, 1850) and L. nesiotes Dall, 1915 have also been reported, but their taxonomic status is not clear.  One or both may be variants of L. californica.	Lyonsia (Lyonsia) californica
2b	Shell usually at least slightly deformed; valves without radial ribs; chalky rather than pearly externally, and usually with considerable periostracum (valves tending to crack when dried)	3
3a	Valves gaping posteriorly or ventrally; length sometimes exceeding 10 cm	Entodesma navicula (Agriodesma saxicola in Kozloff's key)
3b	Valves not gaping; length up to about 2.5 cm (often in sponge masses) (not likely to be found north of California)	Entodesma pictum

Entodesma navicula
Mytilimeria nuttalli