PROSOBRANCHIATA. 



799 



able number of fossil forms, the earliest representatives of the genus 

 appearing in the Middle Cretaceous rocks. The genus Conorbis, of 

 the Eocene and Oligocene, nearly resembles Conus, but the spire 

 is elevated and pointed, the shell thus becoming biconical. 



Family 51. PLEUROTOMiDiE. — In this family the shell (fig. 713) 

 is spindle-shaped, with an elevated spire and an elongated aper- 

 ture, produced in front into a straight canal. The outer lip has 



Fig. 712. — Conus deperditus. 

 Eocene. 



Fig. it.?,.— Pleurotoma 

 rostra fa. Eocene. 



a slit or sinus posteriorly, near the suture. The principal, or 

 sole, genus comprised in this family is Pleurotoma itself, which 

 attained an enormous development in the Tertiary period, and is 

 still very largely represented. A few forms of Pleurotoma are 

 known to occur in the Cretaceous rocks ; but, according to Zittel, 

 more than nine hundred Tertiary species are known, while over six 

 hundred and fifty recent species exist. The genus Pleurotoma has 

 been divided by conchologists into a number of subordinate groups, 

 which are sometimes regarded as subgenera, sometimes as inde- 

 pendent genera. 



Family 52. Terebrid/E. — This family comprises only the genus 

 Terebra, in which the shell is many-whorled and turreted, and 

 the body-whorl is of small size. The aperture is small, and there 

 is a short canal or notch in front ; while the outer lip is thin and 

 sharp. The living species of Terebra inhabit warm seas ; and the 

 oldest fossil forms appear in the Eocene Tertiary. 



