PROSOBRANCHIATA. 



793 



The members of this family are all marine, and the two principal 

 genera contained in it are Tritonium ( = Triton) and Ranella. The 

 former of these ranges from the Cretaceous period to the present 

 day, while the species of the latter are Tertiary and Recent. 



Family 41. Buccinid/e. — In this family the shell is conical, with 

 a large aperture, which is notched in front, 

 or prolonged into a very short canal, which 

 is reflected so as to produce a sort of varix 

 on the back of the shell anteriorly. All 

 the members of this family are marine, 

 and the three most important genera com- 

 prised in it are Buccinum, JVassa, and 

 Ebiirna. 



The Whelks form the genus Buccinum 

 (fig. 700), distinguished by the ventricose 

 body-whorl, large aperture, and short re- 

 flected canal. Some few species of Buc- 

 cinum are found in the Cretaceous rocks ; 

 but the genus is essentially Tertiary and 

 Recent. 



The genus JVassa (fig. 701) comprises 

 the " Dog-whelks," in which the shell has 

 a general resemblance to that of Buc- 

 ci?ium, but the columellar lip is expanded 



and callous, and generally shows a tooth-like fold near the anterior 

 canal. More than two hundred recent species of JVassa are known, 

 and a large number of Pliocene and Miocene 

 species have been described. In the early Ter- 

 tiary deposits the genus is sparingly represented, 

 and the oldest types appear in the Chalk. 



In the genus Ebur?ia, the shell is umbilicated 

 when young, but the inner lip is callous, and in the 

 adult condition is expanded so as to cover the 

 umbilicus. The genus is wholly Tertiary and 

 Recent. Lastly, the genus Brachytrema includes 

 Jurassic types of the Buccinidce, in which the shell (After Searks Wood.) 

 is turbinated, solid, and of small size, with nod- 

 ulated, ribbed, or cancellated whorls. The columella is smooth, 

 and there is a short oblique canal. By Fischer the genus is doubt- 

 fully referred to the Cerithiidce. 



Family 42. Columbellid/e. — In this family the shell is ovate or 

 fusiform, with a short spire and a large body-whorl. The aperture 

 is long and narrow ; the outer lip is thickened and toothed inter- 

 nally, and the inner lip is toothed or granulated in front. The 

 members of this family are all marine, and the three principal genera 



Fig. 700. — Bziccinum glaciale 

 Post-Pliocene and Recent. 



Fig. 701. — Nassa 

 />usillzna. Pliocene. 



