MAZATLAN UNIVALVES 357 
409. Risso1Ina 
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. 
Hab.—Mazatlan ; extremely rare, off Spondylus ; Z’pool Col. 
—, sp. ind. 
410. Rissoina WoopwakDit, 2. s. 
R.t. minore, elongatda, angusta, albidd, interdum alabastro 
simili; anfr. vii.-vill. subplanatis, quorum i. primi leves, 
tumidiores ; suturad impressad ; marginibus spire excurvatis ; 
costis xil.-xiv. in anfr. utroque angustis, acutioribus, 
lineis declivibus apicem versus ascendentibus ; aperturam versus 
sepe crebrioribus ; t. guniore ad basin elongatam evanidis, 
seniore basin subelongatam amplectantibus ; interstitiis latis, 
concavis, interdum minutissime striulatis, striulis costibus 
parallelis ; aperturaé normali ; axi t. juniore producta, 
subcanaliculata, seniore subemarginaté, plicd sew lined spirali 
nulla ; labio solidiore. 
Gono, pee clandestina, C. B. Ad. Pan.’ Shells, p. 177, 
no. 243. 
Comp. Rissoa firmata, C. B. Ad. loc. cit. no. 244. 
Comp. Rissoa Bryerea, Mont. in Forbes § Hanl. Br. Moll. 
vol. ii. 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 strie are strong enough to be seen 
in dead specimens. It appears about the size of R. clandestina, 
but agrees better in sculpture with R. 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 Jong, °123, 
long. spir. °08, lat. °053, div. 24°. 
Hab.—Mazatlan ; rare, on Chama and Spondylus ; L’pool Col. 
