MAZATLASr UNITALVES 323 



state ; but some specimens shew that in this species they are 

 often compact in the very young state, and become looser 

 afterTvards. The variety 'compactum', though smaller than 

 the typical condition, may be a dwarfed form. The species is 

 known by the great prominence and squareness of the ribs and 

 interstices ; the compact growth ; and the plug, which is 

 nearly flat over the surface, gradually rising in a curve to the 

 little finger-shaped mucro, which is blunt at the top, short, and 

 placed somewhat to the right. An extraordinarily large sp. 

 measures long. 'QI7'2, led. (interstices) "016, (outside) '02. 



Sah. — Mazatlan ; rare, in Spondylus and Chama ; L'loool Col, 



Tablet 1529 contains 7 specimens, typical form, in different 

 conditions of growth, one of which has the operculum in situ. — 

 1530, 7 do. var. compactum, and intermediate forms. The 

 youngest specimen is somewhat doubtfully afHliated. 



371. C^CTTM UNDATTTM, ? 71. S. 



C. (Anellum) t.juniore exili, gracile tereti,plus minusve arcti' 

 atd, Icevi, nitidd, aperUira hand declivi ; testa adiilta curtd, 

 obesd ; extremitatibus ad angulum circ. 115° alterd ad alteram 

 truncatis, utrinque contractu, vix annulatis ; aliter, super- 

 ficie eleganter undatd, annulis (numero variantihus x. — xv.^ 

 acutis, 'plus ininusve extantibtis, interdum vix monstrantibus ; 

 interstitiis concavis, Icevilms ffaperturaprimum contractd, deinde 

 paululuni reflexd, acuta ; se2}to tijpice ungidato ; lateraliter onar- 

 ginihus rectis, triangulum scalenumformantihus, apice laterali, 

 acuta, prominente ; ad, frontem regulariter excurvato, tumente. 

 Operculo extus concavo, mtdtispirali, anfr. circiter xv. suturd 

 distinctd, margine simplici. 



Adolesc. .? = C. parvum, C. B. Ad. Pan. Shells, pp. 163, 311 : 

 "t. arcuatd, gracili, vix tereti ; albidd ; annulis xv. acutis, 

 distantibus, haud multum elevatis : apice laterali, prominente. 

 Long. -065, lat. -019." Sp. un. 



The description of C. B. Adams' unique specimen being 

 imperfect, I did not feel at liberty to affiliate the very numerous 

 series of Mazatlan shells, merely because the ribs were sharp, 

 to a species whose name is applicable to the whole genus. The 

 specimen is however most likely the adolescent state of this 

 species, after the waved rings have been formed, but before 

 the final contractions. The yoimg state of this shell cannot be 

 distinguished from that of the nest species. It is long, slender 



