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 

Platyacra impressa, 
Schafh. sp. Lower 
Lias; Hochfellen, 
Bavaria. Cirrus nodosus, Sowb. Lower Oolite ; Yeovil, England. 
Fig. §31. 
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, vy. 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 Recent. 
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 

Fic. 833 
Fic. $32. Prodhus techies ial: Microdoma, Meek and Worthen, from the 
Turbina spiralis, sanus, Brongt. Oligo- Carboniferous; Turbina (Fig. 832) and Tur- 
Miinst. Keuper; St. cene ; Castel Gomberto, : ae Se: 5 a 
Cassian, Tyrol. tay ate honellina, de Koninck, ranging from the Car- 
boniferous to the Trias. 
Trochus, Linn. (Fig. 833). Shell conical or pyramidal ; whorls slightly convex or 
