452 MOLLUSCA SUB-KINGDOM VI 
Koninck, from the Carboniferous ; Chrysostoma, Swainson (Fig. 844), from the Jura, and 
others, which are probably the ancestors of the Umbonidae. 
Whether the genera Teinostoma (Fig. 845) and Vitrinella, Adams, together with 
their fossil allies from the Carboniferous onward, are rightly assigned to this group, is 

Fic. $44. 
Chrysostoma 
Fic. $45. Fic. 847. 
Fic. $46. 
Aemon, d’Orb. Teinostoma rotellae- Adeorbis _ tricostatus, 
sp. Middle jormis, Desh. Calcaire Helicocryptus pusillus, Roem. sp. Desh. Eocene (Middle 
Jura; Balin, Grossier ; Grignon, Coral- Rag; Lindener Berg, near Meeressand) 3; Auvers, 
near Cracow. near Paris. Hannover. Seine-et-Oise. 
doubtful.  Helicocryptus, VOrb. (Fig. 846), from the Jura and Cretaceous, is related to 
Vitrinella. Cyclostrema, Marryat, comprising small, lustrous shells; and the spirally 
striated ones known as Adeorbis, S. Wood. (Fig. 847), present strong resemblances to the 
Umbontidae ; but, according to Fischer, they form separate families. Both of the last- 
named genera have fossil representatives in the Tertiary. 
Family 13. Neritopsidae. Fischer. 
Shell oval to semi-globose, with short, sometimes laterally twisted spire, and without 
umbilicus or nacreous layer. Body whorl very large; aperture oval or semi-circular. 
Inner lip callous, curved, and occasionally notched. Operculum calear- 
A, eous, not spiral, with sub-central nucleus, and internally with callous 
columellar margin, which forms a broad, angular, or rounded process 
in the middle. Devonian to Recent. 
The Neritopsidae are distinguished from the closely related 
Fic. 848. Neritidae, principally by the totally different, non-spiral operculum, 
tales aera a and by the fact that the earlier whorls are not resorbed, as in the 
Keuper; St.Cassian. latter family. Detached opercula have been described under the 
names of Peltarion, Scaphanidia, Cyclidia, and Rhynchidia. 
Naticopsis, M‘Coy (Neritomopsis, Waagen), (Figs. 848-850). Shell oval to globose, 

Fic. 849. Fic. 850. 
A, Naticopsis ampliata, Phil. Carboniferous; Visé, Belgium. Naticopsis lemniscata, Hoernes. Trias ; 
B, Operculum of N. planispira, from same locality (after de Esino, Lombardy. Original coloration 
Koninek). preserved. 
smooth or transversely striated. Aperture oval; inner lip flattened, somewhat 
