320 MAZATLAN UNIVALVES 
very large swelling previous to the aperture. It measures 
long. 112, lat. (juxta aperturam) *025~-"028. 
Of the typical form about a dozen sp. were found, but most 
of them were rubbed. They present however the different stages 
of growth. The youngest measures long. ‘06, Jat. ‘O0O8—'017. 
The largest Te » L038 ,, °“O1b—'023. 
A fragment (? conspecific) 53 ss ‘027 
Hab.—Mazailan ; extremely rare, off Spondylus; L’pool Col. 
Tablet 1525 contains 4 sp. and fragment of different ages, 
typical form. 
Tablet 1526 contains 2 sp. ?var. semileve. 
367. CECUM SUBIMPRESSUM, 2. $ 
C. (Anellum) ¢. elongata, haud parva, solidiore; annulis 
creberrimis, rotundatis, haud extantibus, interstitis parvis, 
sepius subimpressis ; septo mucronato; margine laterali viz 
concavo ; mucrone parva, obtusd, ad dorsum sitd; operculo 
? concavo. 
Of this species only 2 adult and 6 young specimens were 
found; these, being in different stages, have to be retained. 
The sculpture and habit of growth resemble those of C. elonga- 
tum ; from which it, differs especially in the narrow mucro. 
The only specimen possessing the operculum is of aberrant 
growth. The sutures are least impressed in the very earliest 
stages. The smallest sp. measures Jong. ‘058, lat. “OO8—'016. 
The largest ‘ nie ldoe ee Olgas Oe 
Hab.—Mazatlan ; extremely rare, off Spondylus ; L’pool Col. 
Tablet 1527 contains 7 specimens in different stages of growth, 
of which one possesses the operculum, and another is curiously 
deformed. 
368. Czcum Firmatum, C. B. Ad. (diagn. auct.) 
C. (Anellum) t. robustd, arcuatd, satis tereti; aperturd via 
constricta ; t. jumore primum levi; dein annulis circiter 
XXlil. — XXxll., primum acutioribus, dein parum quadratis ; 
interstitiis ia dratee plus minusve distantibus, interdun 
aperturam versus evanidis ; septo subungulato ; apice parum 
prominente, subdextrorsum sito, obtusiore, latiore ; margine ad 
latus subrecto; operculo ?.... 
