MAZATLAN UNIVALVES 419 



minimis, quarum una supi'osuturalis simplex, ef iv. tuberculosce, 

 tuberculis rotundatis, lineis transversis sihi parallelis sitis, in 

 anfr. penult, circiter xx., in t. adultd evanidis ; lirulis in basin 

 Totundatam circiter Vii., t.juniore acutiotnbu-s, interstitiis tenuis- 

 sime et creberrime decussatis ; apertura ovatd, peritremati vix 

 co7itinud ; labro acuto, vix planato, supra suturam- producto, ad 

 basin vix sinuato ; labia in parietem tenuissimo, infra distincto, 

 rimulam umbilicalein formante ; plica intus acuta, extu^ obticsd, 

 hand conspicud. 



This fin& species approaches Monoptygma. It differs from 

 Chr. rotundata in the tuberculation of the spiral ribs and the 

 much greater prolongation of the shell. The basal striae seem 

 to become broader in the adult, as the tuberculation of the 

 upper ribs vanishes. The young shells are slender, with 

 rounded bases. The tubercles are arranged in the form of 

 transverse ribs, parallel to each other, and running in slanting 

 lines to the apex. Long. "153, long. spir. '095, lat. '067. 

 div. 22". 



Hab. — Mazatlan ; 4 sp. and fragment, off" Spondylus ; I! pool 

 Col. 



Tablet 1971 contains the largest sp. presented by R. D. Dar- 

 bishire, Esq. and a very young sheU which may belong to this 

 species. 



Section B. Species typiea. 



507. Chbysallida communis, C. B. Ad. 



Chemnitzia communis, C. B, Ad. Pan. Shells, no. 223, pp. 166, 

 312. 



Chr. t. ovato-oblongd, gibbosd, soUdiore, nitente, albd ; vertice 

 nucleoso parvo, spircB trmicatianem dimidio tegente, anfr. ii. 

 verticaliter sitis, apice vix celato ; marginibus spirce maxime 

 exeurvatis ; avfr. normalibus iv. — vi, planatis, suturdprofundd ; 

 clatkris transversis, utrinque marginibus spirce parallelis, testa 

 juniore conspicuis, aperturam versus evanescentibus, ju-xta 

 peripheriam terminantibus, anfr. penult, circiter xxii. ; liris 

 spiralibus iv. vel V. suprasuturalibus, anfr. primis minoribus, 

 anfr. ultimis scepe clathras in tubercula secantibus ; lirulis 

 rotundatis vix expressis circiter x. supra basin ovatam inter- 

 stitiis punctulis decussatis ; sculpturd in labro tenuissimo, dia- 

 phano evanescente ; labro antice late sinuato, postice planato 

 angustato, ad suturam constricto, labio scepe a pariete separata ; 



