CLASS IV 



GASTROPODA 



531 



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

 of the whorls tlian in tlie upper. Aperture rounded, sometimes witli 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. 



Harausina Gemni. Sinistral, with nodose longitudinal keels, and no umbilicus. 

 Lias. 



Platyacra v. Amnion (Fig. 879). Like the last, but with flattened apex, and the 

 earlier whorls discoidal. Lias. 



Cirrus Sowb. (Scaevola Gemm.) (Fig. 880). 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, turhinate or pyramidalj nacreous internally. Aperture trapezoidal or 

 sub-circular, 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 liave been commonly assigned to 

 the genus Trochus. Among the more ancient 

 true Trochidae may be mentioned the following : 

 the Trochus species described by Lindström 

 from the Silurian of Gotland ; also Flemingia 

 and Glyptohasis de Koninck, and Microdoma 

 Meek and Worthen, from the Carboniferous ; 

 Turhina (Fig. 881) and Turhonellina de 

 Koninck, ranging from the Carboniferous to 

 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) ; Folydonta Schum. ; Clanculus Montf., etc. 



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



Turhina spirulis 

 Münst. Keuper ; St. 

 Cassian, Tyrol. 



Trochus {Tectus) luca- 

 sanus Brongt. Oligo- 

 cene ; Castel Gomberto 

 near Vicenza. 



Fig. 883. 



Monodonta nodosa Münst. 

 Keuper ; St. Cassian, Tyrol. 



Fkj. 884. 



Monodonta (Oxystele) patukt 

 Brocchi. Miocene ; Steina- 

 brunn, near Vienna. 



Fi(i. 885. 



Gibhula picta 

 Eichwald. Mio- 

 cene; Wiesen, 

 near Vienna. 



Fir,. 886. 



Gihhula brocchii 

 Mayer. Pliocene ; 



Montopoli, Tuscany. 



columella ending below in a tooth. 

 Phil., the tooth is wanting. 



Trias to Recent. In the subgenera Osilinus and 



