CLASS IV 



GASTROPODA 



531 



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

 of the whorls than in the iipper. 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. 



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

 Lias. 



Platyacra v. Ammon (Fig. 8Y9). Like the last, but with flattened apex, and the 

 earlier whorls discoidal. Lias. 



Girriis Sowb. (Scacvola Gemm.) (Fig. 880). Sinistral, turbinate shells, deejjly 

 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 ptjramidal, nacreous internally. Aperture trapezoidal or 

 sitb-circidar, peristome disconnected, inner lip often hearing 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. Paleozoic and Mesozoic forms in many cases do not harmonise 

 with recent genera, but represent rather col- 

 lective types, in which characters now distri- 

 buted amongst several genera or even families 

 are united. Shells incapable of more accurate 

 determination have been 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 Worthen, from the Carboniferous ; 



Turbina (Fig. 881) and Turbonellina de 



-- . , . J. ii. r-i 1 -i- i Mtinst. Keuper;St. 



Koninck, ranging from the Carbomterous to cassian, Tyrol. 



the Trias. 



Trochus Linn. (Fig. 882). Shell conical or pyramidal ; whorls slightly convex or 



flat ; base angular at the periphery. Inner lip often truncated anteriorly, thickened 



or with teeth. Silurian to Recent. 



Subgenera : Tectus Montf. (Fig. 882) ; Polydonta Schum. ; Clanculus Montf., etc. 



Monodonta Lam. (Figs. 883, 884). Turbinate, with nearly round aperture, the 



Fig. SSI. 

 Turbiiui spiralis 



Fio. 882. 



Trnelius (Tectus) luca- 

 sanus Biongt. Oligo- 

 cene ; Castel Gomberto 

 near Vicenza. 



Fig. SS3. 



Monodonta nodosa Mtinst. 

 Keuper ; St. Cassian, Tyrol. 



Fig. 884. 



Monodonta {Oxystcle) patula 

 Brocchi. Miocene ; Steina- 

 brunn, near Vienna. 



Fig. 885. 



Gibhula picta 

 Bichwald. Mio- 

 cene ; Wiesen, 

 near Vienna. 



Fig. 8Si5. 



Gibbula hrocchii 

 Mayer. Pliocene ; 



Montopoli, Tuscany. 



columella ending below in a tooth. Trias to Recent. In the subgenera Osilinus and 

 Oxystele Phil., the tooth is wanting. 



