342 MAZATLAN UNIVALVES 
Tablet 1615 contains a very young sp.: the adult above 
measured ; a fragment between the two, and another shewing 
a curiously mended fracture. 
393. TRIFORIS PINFREQUENS, C. B. Ad. 
Pan. Shells, no. 209, pp. 159, 310. 
A few shells are kept separate (till more specimens are 
obtained) as they seem to differ from Tr. inconspicuus in hay- 
ing the apical whirls scarcely sculptured, but soon adopting the 
permanent type. Prof. Adams’ species is only distinguished 
by the number and strength of the three rows of nodules. 
None of the Mazatlan forms have these so large as in the 
British shell. A specimen with 9 normal whirls measures 
long. °18, long. spir. °135, lat. *065, div. 20°. 
Hab—Panama; 2 sp. C. B. Adams.—? Mazatlan ; 6 sp. off 
Chama and Spondylus ; L’pool Col. 
Tablet 1616 contains a young and an adult specimen. 
Genus CERITHIDEA, Swains. 
H. §& A. Ad. Gen. vol. i. p. 292:—v. A. Ad. Monogr. in Proe. 
Zool. Soc. 1854, p. 83. « 
erithium, pars, auct. Shell light, transversely ribbed, gene- 
rally with a widely expanded mouth, rudimentary siphonal 
notch and decollated apex. Operculum multispiral. Animal 
living in brackish mud (Nuttall), or out of the water on 
twigs.* 
394. CeritHipEa Montacnet, D’Orb. 
Cerithium Montagnei, D’Orb. Voy. Am. Mer. Moll. v. 443, 
pl. 63, f. 3, 4. — Kien. Icon. Conch. p. 99, pl. 30, f. 1.—Sow. 
Thes. Conch. p. 888, sp. 171, pl. 186, f. 284-6.— Whe. in Zeit. 
fF. Mal. 1850, p. 178, no. 38.—B. M. Cat. D’Orb. Moll. 
* Clark (Moll. Test. Mar. Br. p. 305) in accordance with a habit of generalizing 
for the world from the Mollusks of the South Devon fauna, surmises that all 
shells with multispiral opercula belong to hermaphrodite animals, and should be 
separated from those of paucispiral elements. If so, Cerithidea must be separ- 
ated from Cerithium, Modulus from Littorina, Aletes from Siphonium, Torinia 
from Solarium, Adamsiella from Cyclostoma, Melanoides from Melania, Skenea 
from Rissoa, Valvata from Paludina, &c. Mr. Clark himself unites the multispirat 
genera Bittium and Skenea to the paucispiral Rissoa. ; 
