MAZATLAN UNIVALVES 185 
Susctass PROSOBRANCHIATA. 
In those families in which the first whirls are abnormal 
as compared with the rest, as compared with the rest, these 
are called the nucleus, although they may be more than the 
part so formed in the egg, and their first whirl the apex. 
The measurements marked div. represent the mean divergence 
of the outlines of the spire. 
Orper HETEROPODA. 
Famity IANTHINIDA. 
This aberrant group is ranked with Carinaria in the Subclass 
Heteropoda by H. & A. Adams, Gen. vol. i. p. 85; between 
Sealaride and Stylinide by Philippi, Handb. Conch. p. 179; 
next to Ampullariade, Gray, 1847; with the Heteropoda in 
Order Pleurobranchiata, Gray 1850; between Neritopside 
and Scalariade, in the Suborder Proboscidifera, Gray, 1856 ; 
next to Trochide, Forbes & Hanl. Br. Moil. vol. uu. p. 547 ; 
in the Family Haliotide, Woodw. Man. pt. 1, p. 148. It has 
affinities in all these groups. The vertex is not sinistral as in 
Pyramidellide ; but dextral, fixed slanting from the future 
axis of the shell. 
Genus JANTHINA, Lam. 
It is necessary that more materials should be collected, and 
the animals observed, before a decided opinion can be expressed 
on the species of this genus. Some would reduce most of the 
forms to one or two species, Dr. Gray even hazarding the 
conjecture that the two forms usually found together (I. fra- 
gilis and I. prolongata) are the sexes of one species, (B. M. Cat. 
D Orb. Moll. p. 35.) Mx. M’Andrew however found them in 
the Canaries separate, and miles apart. Others will believe 
that many different species have been confounded. In the ab- 
sence of information respecting the animals, it has been thought 
best to describe the Pacific species as distinct ; aithough it is 
highly probable that they may hereafter be identified with 
those from the W. Indies. The smaller Atlantic form is repre- 
sented, in the Pacific waters, by the beautiful I. bifida, Nutt. 
from the Sandwich Islands. 
242. IANTHINA STRIULATA, 20m. prov. 
It. “LT. fragili” simillima ; nucleo haud valde oblique sito, 
anfractibus iv., globoso, apice planato, suturis nullis ; anfracti- 
