342 MAZATLAlf UNIYALVE8 



Tablet 1615 contains a very young sp. : the adult above 

 measured ; a fragment between the two, and another shewing 

 a curiously mended fracture. 



393. TEIFOEIS PINFEEQUENS, 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 hav- 

 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. 

 JSTone 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, lot. *065, div. 20. 

 Hob. Panama ; 2 sp. C. IB. Adams. ? Mazatlan ; 6 sp. off 



Chama and Spondylus ; L'pool Col. 



Tablet 1616 contains a young and an adult specimen. 



GENUS CEBITHIDEA, Swains. 



H. fy A. Ad. Gen. vol. i. p. 292 : v. A. Ad. Monogr. in Prog. 

 Zool. Soc. 1854, p. 83. 



Cerithium, 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. CEEITHIDEA MONTAGNEI, D'Orb. 



Cerithium Montagnei, D'Orb. Voy. Am. Mer. Moll. v. 443, 

 pi. 63, f. 3, h Kien. Icon. Condi, p. 99, pi. 30, f. 1. Sow. 

 Thes. Conch, p. 888, sp. 171, pi. 186, f. 284-6. Mke. in Zeit. 

 f. Mai. 1850, p. 178, no. 38. B. M. Cat. IT Orb. Moll. 



* Clark (Moll. Test. Mar. Sr. 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 Kissoa, Valvata from Paludina, &c. Mr. Clark himself unites the multispiral 

 genera Bittium and Skenea to the paucispiral Kissoa. 



