MAZATLAN UNIVALVES ae 
state ; but some specimens shew that in this species they are 
often compact in the very young state, and become looser 
_afterwards. The variety “compactum’, though smaller than 
‘the typical condition, may be a dwarfed form. The species 1s 
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. ‘072, lat. (interstices) ‘016, (outside) *02. 
Hab.—Mazatlan ; rare, in Spondylus and Chama; L’pool 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 affiliated. 
371. CacuM UNDATUM, ?2. s. 
C. (Anellum) t. juniore exilt, gracile tereti, plus minusve arcu- 
até, levi, nitidad, apertura haud declivi ; testa adultd curté, 
obesa ; extremitatibus ad angulum cire. 115° alterd ad alteram 
truncatis, utringque contractis, vix annulatis ; aliter, super- 
jicie eleganter undaté, annulis (numero variantibus x.—xv.) 
acutis, plus minusve extantibus, interdum via monstrantibus ; 
interstitiis concavis, levibus ; apertura primum contractd, deinde 
paululum reflexa, acuta ; septo typicé ungulato ; lateraliter mar- 
ginibus rectis, triangulum scalenum formantibus, apice laterals, 
acuto, prominente ; ad frontem regulariter excurvato, tumente. 
Operculo extus concavo, multispirali, anfr. circiter xv. sutura 
distinctd, margine simplict. 
Adolesc. 2=C. parvum, C. B. Ad. Pan. Shells, pp. 163, 311: 
“t. arcuatd, gracili, via tereti; albidé; 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 young state of this shell cannot be 
distinguished from that of the next species. It is long, slender 
