416 



MAZATLA^ST UNIVALVES 



ornatd, quarum prima ultimam anfractus superioris tegit ; 

 carind quarto, in basin angulatam sitd ; aperturd ovali ; labro 

 solido ; plied parvd, celatd. 



This stout little shell is at once recognized by its strong 

 keels, of which two appear on the penultimate whirl, the third 

 being covered by the first of the ultimate. Lone/. '252, long, 

 spir. '03, lat. '032, div. 35. 

 j5fo5. Mazatlan ; 2 sp. off Chama ; L'pool Col. 



Tablet 1966 contains the largest specimen. 



502. PARTHENIA ZIZIPHINA, n. s. 



.P. t. conicd, albd ; vertice nucleoso mammillato ; anfr. liris 

 rotundatis v. spiralibus, quarum una major ad peripheriam, 

 duce in spird, duce in basi angulatd sites sunt ; plied transversd. 



Tablet 1967 contains a small Ziziphinus-shaped shell, very 

 young, differing from P. exarata in the number and strength 

 of the spiral ridges. Long. '026, lat. '022. 



Hab. Mazatlan ; 1 sp. on Chama ; L'pool Col. 



SUBGENUS CHKYSALLIDA. 



Testa utrinque constricta, pupiformis ; peritrema continuum, 

 ad basin undat&m ; labrum juxta aperturam tenue, intus soli- 

 dius ; plica columellaris declivis, scepe celata ; superficies ple- 

 rumque cancellata. Operculum \_sp. typ.~] radiatim corrugatum. 

 Chemnitzia, pars, C. J8. Adams, (Pan. Shells, no. ?220, ?222, 



223, 226.) 



The typical species differ from Chemnitzia in having a dis- 

 tinct, though hidden plait ; from Odostomia proper in having 

 the peritreme continuous ; from Monoptygma in the Pupiform 

 growth, contracted at each end ; and from all in the labrum, 

 which is extremely thin at the edge, in the adult as well as in 

 the young shell ; while within, as shewn by broken specimens, 

 it is tolerably strong. That the sectional limits are not exactly 

 defined, is true of almost every group in this family. Similar 

 forms occur both in the E. and W. Indies. The Mazatlan 

 shells are represented by 15 species, of which about 650 speci- 

 mens were found. The specific characters often depend on 

 very minute differences. 



