OPISTHOBRANCHIATA. 



SOI 



Fig. 714. — Cinulia ave liana. Chalk. 



generally plaited. The living examples of this family are mostly 

 small and thin-shelled, but some of the fossil forms are of consider- 

 able size and are thick-shelled. The family attains its maximum in 

 the Secondary period. 



In the genus Actceon (Tomatella) the shell is ovate, with a well- 

 marked spire, the outer lip thin, and the columella with one or more 

 strong folds. The genus ranges 



from the Trias to the present day. 

 In Actceonina, ranging from the 

 Carboniferous Limestone to the 

 present day, the shell is elongated, 

 and the columellar lip is without 

 folds. In Actceonella the shell is 

 ventricose, with a very large body- 

 whorl, and the columella shows 

 three strong oblique folds in front. This genus is characteristic of 

 the Middle and Upper Cretaceous deposits, having a remarkably 

 wide range in space, and being sometimes represented by very 

 numerous individuals in particular zones. 



In the genus Cinulia (fig. 714) the shell is globular, with a small 

 spire ; the outer lip is reflected and crenulated interiorly ; and the 

 columella exhibits toothlike folds. The genus is exclusively con- 

 fined to the Cretaceous rocks. In Cylindrites, again, the shell is 

 cylindrical, and smooth, with an elongated aperture and a fold on 

 the columella. The genus ranges from the Trias to the Cretaceous 

 rocks. 



Lastly, the genus Ringicula (fig. 715) is now 

 usually placed in this family. The shell in this 

 genus is ventricose, with a small spire, the colum- 

 ella callous and deeply plaited, and the outer lip 

 thickened and reflected. The earliest forms of 

 Ringicula appear in the Cretaceous rocks, but 

 the genus attains its maximum in the later Tertiaries and at the 

 present day. 



Family 2. Bullid^e. — In this family the shell is convoluted and 

 thin, with a sharp outer lip and an elongated 

 aperture. The spire is small or concealed, 

 and the shell is often more or less complete- 

 ly invested by the soft parts of the animal. 

 A large number of fossil forms of this family, 

 beginning in the Trias, are known, and the 

 group has attained its maximum at the pres- 

 ent day. The three principal genera are Bulla, Scaphander, and 

 Cylichna. 



The genus Bulla (fig. 716) comprises the so-called " Bubble- 

 vol. 1. 3 e 



Fig. 715. — Ringicula 

 ventricosa. Pliocene. 

 (After Searles Wood.) 



Fig. 716. — Bulla suf>ra- 

 jurensis. Middle Oolites. 



