450 



MOLLUSCA 



SUB-KINGDOM VI 



pyramidal, with deep sutures, and no umbilicus. Spiral keels usually nodose or spiny, 

 and crossed by strong transverse striae, which are more numerous in the lower portion 



FIG. 



Platyacra impressa, 

 Schafh. sp. Lower 

 Lias; Hochfelleri, 

 Bavaria. 



FIG. 831. 

 Cirrus nodosus, Sowb. Lower Oolite ; Yeovil, England. 



of the whorls than in the upper. Aperture rounded, sometimes with a shallow notch. 

 Trias to Cretaceous ; common in all divisions of the Jura. 



Oncospira, Zitt. Pyramidal, spirally ribbed, with one or two transverse swellings 

 on each whorl, disposed continuously along the spire. Jura. 



Hamusina, Gemm. Sinistral, with nodose longitudinal keels, and no umbilicus. 

 Lias. 



Platyacra, v. Ammon (Fig. 830). Like the last, but with flattened apex, and the 

 earlier whorls discoidal. Lias. 



Cirrus, Sow. (Scaevola, Gemm.), (Fig. 831). Sinistral, turbinate shells, deeply 

 and widely umbilicate. Spire acuminate ; whorls spirally keeled and striated, and 

 with strong transverse ribs. Trias to Middle Jura. 



Family 11. Trochidae. Adams. 



Shell conical, turbinate or pyramidal, nacreous internally. Aperture trapezoidal or 

 sub-circular, peristome disconnected, inner lip often bearing a tooth. Base more or less 

 flattened; operculum thin, horny. Ordovician to Eecent. 



Precise determination of the numerous fossil Trochidae is not less difficult than 

 that of the Turbinidae. Palaeozoic and Mesozoic forms in many cases do not har- 

 monise with recent genera, but represent 

 rather collective types, in which characters 

 now distributed amongst several genera or even 

 families are united. Shells incapable of more 

 accurate determination are commonly assigned 

 to the genus Trochus. Among the more 

 ancient true Trochidae may be mentioned the 

 following : the Trochus species described by 

 Lindstrom from the Silurian of Gotland ; also 

 Flemingia and Glyptobasis, de Koninck, and 

 Microdoma, Meek and Wort-hen, from the 

 Carboniferous; Turbina (Fig. 832) and Tur- 

 bonellina, de Koninck, ranging from the Car- 

 boniferous to the Trias. 

 Shell conical or pyramidal ; whorls slightly convex or 



Fio. 832. 



Turbina spiralis, 

 Miinst. Keuper ; St. 

 Cassian, Tyrol. 



FIG. 833. 



Trochus (Tectus) Luca- 

 sanus, Brongt. Oligo- 

 cene ; Castel Goinberto, 

 near Vicenza. 



Trochus, Linn. (Fig. 833). 



