530 



MOLLUSCA 



PHYLUM VI 



Family 10. Trochonematidae Zittel. 



Shell pyramidal, turhinate or discoidal, dextral or sinistral, with internal nacreons 

 layer. JVhorls convex, with one or more longitudinal Jceels, and slightly undulating 

 transversa Striae or ribs. Aperture rounded, sometimes with faint notch. Operculum 

 unknown, presumahly horny. Marine. Cambrian to Cretaceous. 



This extinct group is very abundant in tlie Paleozoic, and notably so in Jurassic 

 rocks. The shells, as a rule, are highly ornamented, and have been associated by some 

 with the Littorinidae, by others with the Turbinidae or Purpurinidae. They form 

 a distinct family, which is best placed in the neighbourhood of the Turbinidae and 

 Trochidae. 



Trochonema Salter. Pyramidal to turbinate, deeply umbilicate, longitudinally 

 keeled and transversely striated. Aperture round ; the umbilicus surrounded by a 

 keel. Cambrian to Silurian. 



Eunema Salter (Fig. 876). Pyramidal, with acute, elongate spire, and no umbilicus. 

 Whorls with two or more spiral keels, and strong transverse Striae. Aperture oval, 

 slightly notched anteriorly. Ordovician to Devonian. 



Fig. 876. 



Eunema strigilata Sal- 

 ter. Ordovician ; Pau- 

 quette Falls, Canada. 



Fig. 877. 



Cydonema bilix Conrad. 

 Ordovician ; Cincinnati, 

 Ohio. 



Ambcrleya capitanea 

 Münst. Upper Lias ; La 

 Verpilliere, near Lyons, 

 France. 



Cydonema Hall (Fig. 877). Turbinate, whorls infiated and ornamented with fine 

 Spiral Striae ; aperture rounded, peristome discontinuous. Ordovician to Devonian. 



Strophostylus and Holopea Hall ; Gyronema Ulrich. Ordovician and Silurian. 

 Dyeria Ulrich. Ordovician. Bucanospira Ulrich. Silurian. 



Amberleya Morr. and Lyc. (Eucyclus Deslongch.) (Fig. 878). Turbinate to 



Fig. 879. 



Platyacra impressa 

 (Schafhautl). Lower 

 Lias ; Hoclifellen, 

 ßavaria. Cirrus nodosus Sowb. 



pyramidal, with deep sutures, and no umbilicus, 



Fio, 880. 

 Lower Oolite 



Yeovil, England. 



Spiral keels usually nodose or spiny. 



