MAZATLAN UNIVALVES 185 



SUBCLASS PEOSOBEANCHIATA. 



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. 



OEDER HETEEOPODA. 

 FAMILY IANTHIJNTD.E. 



This aberrant group is ranked with Carinaria in the Subclass 

 Heteropoda by H. fy A. Adams, Gen. vol. ii. p. 85 ; between 

 Scalaridse and Stylinidse by PJiilippi, Handb. Conch, p. 179 ; 

 next to Ampullariadse, Gray, 1847 ; with the Heteropoda in 

 Order Pleurobranchiata, Gray 1850 ; between Neritopsidse 

 and Scalariadae, in the Suborder Proboscidifera, Gray, 1855 ; 

 next to Trochidse, Forbes fy Hani. Br. Moll. vol. ii. p. 547 ; 

 in the Family Haliotidse, Wbodw. Man. pt. 1, p. 148., It has 

 affinities in all these groups. The vertex is not sinistral as in 

 Pyramidellidse ; but dextral, fixed slanting from the future 

 axis of the shell. 



GENUS IANTHINA, 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, (J3. M. Cat. 

 D'Orb. Moll. p. 35.) Mr. 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 ; although 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. 



J- 242. IANTHINA STEIULATA, nom.prov. 



I. t. "I.fragili" simillima ; nucleo haud valde oblique sito, 

 anfractibus iv., globose, apice planato, suturis nullis ; anfracti- 



