392 MANUAL OF THE MOLLUSCA. 
alive. The shell is horny and flexible, and always of a greenish 
colour. 
Distribution, 16 species. India, Philippines, Moluccas, Aus- 
tralia, Feejees, Sandwich Islands, West America. 
Fossil, 91 species. Lower Silurian—. North America, Europe, 
Thibet. 
Lingule existed in the British seas as late as the period of the 
Coralline Crag. The recent species have been found at small 
depths, and even at low water half buried in sand. LZ Davisit, 
Lower Silurian, Tremadoc, has a pedicle-groove like Obolus, 
Fig. 205. (Salter.) 
Fig. 205. Ventral valve. Fig. 206. Dorsal valve. 
Obolus Davidsoni. (Salter.) Wenlock limestone, Dudley. 
A, posterior adductors; B, adjustors; C, anterior adductors. 
The pedicle-scar in the centre of Fig. 168 has no letter. 
Oxouus, Hichwald. 
Synonyms, Ungula, Pander; Aulonotreta, Kutorga. 
Etymology, obolus, a small Greek coin. 
Type, O. Apollinis, Hichw. 
Shell orbicular, caleareo-corneous, depressed, sub-equivalye, 
smooth; hinge-margin thickened inside, and slightly grooved 
in the ventral valve; posterior adductor impressions separate ; 
anterior pair sub-central; impressions of adjustors lateral. 
Fig. 205, 206. (After Davidson.) 
Fossil, 8 species. Lower and Upper Silurian. Sweden, 
Russia, England, United States. 
