36 LECTURES ON MOLLUSC A. 



the species of the shell. The operculum of Cassis is very long and 

 narrow, like that of the Buccinum drawn out ; but in the swollen hel- 

 mets (Bezoardicd) it is shaped like an expanded fan, with the nucleus 

 on the inner margin. The shells of this group seldom make a varix 

 except when mature ; and the pillar lip is thin, seldom plaited. In 

 Levenia (peculiar to west tropical America) the outer lip is sharp, 

 but thickened within ; the operculum being very small, to suit the 

 contracted aperture. In Cyprcecassis, there is no operculum ; the 

 mouth is narrow and toothed on each side like the Cowries ; and the 

 inner lip is very thick, but not projecting as in the true helmets. In 

 Cassidaria, (a genus almost confined to the Mediterranean,) the shell 

 is like Bezoardica, but the canal is only partially bent back : in Sconsia 

 it is not bent back at all. In Oniscia, the canal is straight, and the 

 inner lip wrinkled : while Pachybathron is even more like a Cowry 

 than Cyprcecassis , having the mouth toothed as in Trivia with a notch 

 at each end. The Helmets first appear in the Eocene tertiaries ; but 

 their maximum development, as in most other predacious Grasteropods, 

 is in the existing age. 



Family DOLIAD^;. (Tun Shells.) 



The Tuns are nearly related to the Helmets, both in animal and 

 shell. The latter is always very thin and ventricose, with spiral ribs, 

 and a sharply notched aperture. The animal is large, with a very 

 capacious foot, truncated in front, which it swells out with water when 

 swimming. The head is thick, with the eyes on little stalks at 

 the base of the tentacles. The proboscis is stout and long, and armed 

 with a powerful prehensile collar at the end. The breathing canal is 

 turned back, as in the Helmets. In Dolium, the mouth of the shell, 

 is very wide and open : in Malea, it is curiously contracted, with ribs 

 on each side. The Malea ringens is a very characteristic shell of 

 Pacific tropical America. Its fossil remains, discovered by Dr. New- 

 bery on the Atlantic coast, prove that the two oceans have been sepa- 

 rated since the creation of the species. The Tuns make their appear- 

 ance in the Miocene age. 



Family TRITONIDJE. (Trumpet Conchs.) 



The Tritons were naturally associated with the Murices by concholo- 

 gists ; the only differences observed in the shells being purely arti- 

 ficial, viz : that in Murex the varices (or old mouths) are any number 

 from three to thirteen ; while in this family they are two or one and 

 a half. This trifling distinction, however, is found to be coordinate 

 with an essential difference in the dentition ; the Tritons being in that 

 respect closely related to the Helmets and the Naticas. They differ 

 from the previous families in having but a small foot, and a [nearly 

 straight siphon, inclosed in the canal of the shell. They are almost 

 confined to tropical seas, and have a much greater love for the old 

 world than the new. All the shells of the family have the outer lip 

 toothed within, and most of them have the pillar lip similarly orna- 

 mented. The operculum is generally as in the Muricids. 



