MAZATLAN BIVALVES 125 
ventrali incurvo; epidermide fusca, rugis concentricis parte 
posteriore instructa ; incrustatione in parte posticad solidd, im 
lineas duas ab umbonibus decurrente, striis sagittiformibus incon- 
spicuis ad marginem directis; in rostra solida prolongata, 
appressa, maxima parte non excavata sed ad 
apicem alte effossa, quasi calycem adhibentia. 
The only specimen found is in shape like L. arist. tumidior, 
but differs in the remarkable character of the inerustation. 
This lies in a solid triangular layer over the posterior part, 
with arrow-headed lines pointing away from the vertex of the 
triangle. At the sides, the coarse ruge of the epidermis are 
visible, ending in a diagonal line bounding the posterior part. 
The incrusting beaks -are appressed, as in L. attenuatus, but 
are not hollowed within, as in that species, until the extremity, 
where there suddenly appears a deep cup, dividing the ter- 
mination into two knobs. This might at first appear as if bored 
into by another mollusk, but (1) the excavation is not sideways 
but from the outer end; (2) the remains of the animal are 
fresh within ; (3) the lines of growth on the incrustation dis- 
play a simHar outline. Long. °36, lat. °14, alt. °15. 
Hab.—Mazatlan ; 1 sp. in Spondylus ealcifer; L’pool Col. 
Tablet 571 contains the specimen. 
175. LitHopHacus PLUMULA, Hanl. 
Proc. Zool, Soc. 184A, p. 17. 
Modiola plumula, Hanl. Deser. Cat. p. 239. 
Comp. L. levigatus, B. 1. non Quoy & Gaim. (Cape Upstart, 
Jukes.) 
Comp. L. rugiferus, Dkr. in lit. Mazatlan (teste Cuming) : 
“differt a L. plumula forma et incrustationis indole diversa. 
Margines cardinalis et basalis haud parallel, pars anterior 
minus inflataest. Latus basale rugis instructum est.” Dun- 
ker. Spec. unic. in Mus. Cum. no. 172. 
The species is known by the remarkable character of the 
merustation, which for the most part presents the arrange- 
ment of a feather in lines running out on each side froma 
midrib which joins the umbo to the posterior end. The in- 
crustations form beaks beyond the shell, appressed but not 
prolonged or hollowed within. Its texture is sometimes 
tolerably solid, sometimes in a branching network. It does 
not present an organized structure under the microscope, nor 
