416 MAZATLAN UNIVALVES 



ornatd, quarum prima ultimam anfractus superioris tegit ; 

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

 solido ; plica parvd, celatd. 



This stout little shell is at once recognised by its strong 

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

 being covered by the first of the ultimate. Long. "252, long, 

 spir. -03, lat. -032, div. 35°. 



Hah. — JMazatlan ; 2 sp. off Chama ; L'pool Col. 



Tablet 1966 contains the largest specimen. 



502. Paethenia ziziphina, n. s. 



P. t. conicd, alhd ; vertice nucleoso mammillato ; anfr. tins 

 rotundatis v. spiralibus, quorum una major ad peripheriam, 

 ducB xn ispird, dua; in hasi angulatd sites sunt ; plied tronsversd. 



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. 



Sab. — ]VIazatlan ; 1 sp. on Chama ; L'pool Col. 



Subgenus CHEYSALLIDA. 



Testa utrinque constricta, pupiformis ; peritrema continuum, 

 ad basin undatum ; labrum juxta aperturam tenue, ititus soli- 

 dius ; plica columeUaris declivis, scepe celato ; superficies ple- 

 rumque cancellata. Operculum \_sp. tijp.'] radiatim corrugatum. 



Chemnitzia, pars, C. B. 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 jMonoptj'gma in the Pupiform 

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

 which is extrcmelj- 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 ^lazatlan 

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

 mens were found. The specific characters often depend on 

 very minute differences. 



