MAZATLAN UNIVALVES 357 



409. RISSOINA , sp. ind. 



Tablet 1705 contains a very young shell and a fragment of 

 adult, of large size, differing from R. stricta in the extreme 

 sharpness of the ribs. It appears to be most minutely striu- 

 lated transversely. 

 Hob. Mazatlan ; extremely rare, off Spondylus ; L'pool Col. 



J^ 410. RlSSOINA WOODWARDII, n. S. 



R.t. minor e, elongatd, angustd, albidd, Inter dum alabastro 

 simili ; anfr. vii.-viii. subplanatis, quorum iii. primi laves, 

 tumidiores ; suturd impressd ; marginibus spirce excurvatis ; 

 costis xii.-xiv. in anfr. utroque angustis, acutioribus, 

 lineis declivibus apicem versus ascendentibus ; aperturam versus 

 sape crebrioribus ; t. juniore ad basin elongatam evanidis, 

 seniore basin subelongatam amplectantibus ; interstitiis latis, 

 concavis, interdum minutissime striulatis, sir lulls co stibus 

 par all el Is ; aperturd normali ; axl t. juniore producta, 

 subcanaliculatd, seniore subemarginatd, plica seu lined spirali 

 nulld ; labio solidiore. 



Comp. Eissoa clandestina, C. S. Ad. Pan. Shells, p. 177, 



no. 243. 



Comp. Bissoa firmata, C. S. Ad. loc. cit. no. 244. 

 Comp. Bissoa Bryerea, Mont, in Forbes fy Hani. Br. Moll. 



vol. iii. p. 149. 



The Mazatlan species (of which about 70 adult specimens 

 were found, and as many young and fragmentary,) is very 

 uniform in its characters ; and is distinguished by its small 

 size, shape and distant ribs, (generally 13 on the penultimate 

 whirl,) entire absence of spiral sculpture, and most minute 

 transverse striulation, which is generally seen in the young 

 shell, but very rarely in the adult, and then only under a good 

 microscope. In the pseudo-British species, of which even 

 Montague's small [?] variety is much larger, the ribs are much 

 closer, and the transverse striae are strong enough to be seen 

 in dead specimens. It appears about the size of R. clandestina, 

 but agrees better in sculpture with E. firmata. It is not con- 

 specific with any of the similar E. or W. Indian species that I 

 have been able to subject to the ordeal of the microscope. An 

 unusually large sp. of the Mazatlan shell measures long, *123, 

 long. spir. '08, lat. '053, div. 24. 



b. Mazatlan ; rare, on Chama and Spondylus ; L'pool CoL 



