358 MAZATLAN UNIVALVES 
Tablet 1706 contains a series of 11 specimens of different 
ages, beginning from ‘027 by ‘018, and presenting the extremes 
of variation.—1707, 2 sp. alabastrine variety, and as a contrast, 
one with black incrustation ; also 2 fragments to illustrate the 
adolescent and the adult basal sculpture. 
Genus RISSOA, Préminville. 
Bull. Soc. Philom. 1814, p. 7. 
Loxostoma, Bivon. 1832.— Lamarckia, Leach. — Gonostoma, 
Mihif. 
411. P Rissoa LIRATA, 7. s. 
? R. t. conoided, albido-fuscd, solidiore ; anfr. vi., quorum il. 
primi tumidiores, levi; reliqui planati, eleganter lirati, liris 
plurimis (in spird circiter x.) extantibus, interstitiis equantibus, 
quadratis ; sulco latiore circa peripheriam, suturam superante ;- 
anfr. radiatim obsolete undatis, prope suturam subnodosis ; 
marginibus spire excurvatis ; basi subangulato ; apertura sub- 
ovali, peritremati continuo; labro antice expanso: operculo 
solidiore, corneo, rufo-fusco, semilunato, convexo ; elementis con- 
centricis ; nucleo subcentrali, marginem interiorem versus ; intus 
processu. conico anteriori prelongo, extante, costa ad alterum 
marginem decurrente. 
Of this beautiful shell one perfect specimen was found with 
its operculum. This makes its generic position doubtful; as 
it presents the concentric elements of Jeffreysia and the 
anteriorly projecting process of Rissoina, in conjunction with 
a shell typically Rissoid. The nucleus is projecting, and 
situated about half way between the centre and the inner 
margin. It appears that the entire sculptured layer may be 
abraded ; for a specimen was found (whieh but for one in an 
intermediate state would surely have been regarded as a dis- 
tinet species) without either lire or transverse waves, and 
scarcely shewing the sutural furrow ; but with a smooth surface 
most delicately corrugated transversely. Long. ‘12, long. 
spir. 073, lat. ‘063, div. 25°. 
Hab.—Mazatlan ; 9 sp. off Chama and Spondylus ; L’pool Col. 
Tablet 1708 contains the perfect and the abraded specimens. 
