772 



DIVISIONS OF THE GASTROPODA. 



The name of Coelocentrus ( = Cirrus, De Koninck) is given by 

 Zittel to certain Palaeozoic types which have a general resemblance 

 to Euomphalus, the shell being a flat spiral, with a large umbilicus 

 (fig. 662). The shell differs from that of Euomphalus proper in 

 the fact that the mouth is round, the outer lip is not sinuated, the 



Fig. 660. — Schizostojna (Ophileta) Bella (Billings'). Different views of a nearly perfect 

 specimen. Quebec Group (Ordovician). 



whorls are more or less rounded, and the volutions support one or 

 two rows of prominent spines. 



Discohelix includes discoidal shells, concave on both faces, with 

 the whorls flattened at the circumference, and furnished with two 

 marginal keels, the aperture being quadrangular. Forms of this 

 type appear to be principally characteristic of the Upper Trias and 



Ecculiomphalus distans. Quebec Group (Ordovician). 



of the Jurassic rocks. Bifrontia (Eocene) appears to be nearly 

 related to Discohelix. 



The remarkable Palaeozoic genus Madurea (fig. 663) may be 

 provisionally placed here ; though its precise affinities are very 

 doubtful, and it is regarded by Fischer as probably related to the 



