240 



MANUAL OF THE MOLLUSCA. 



metrical, one of them dividing; protractor sliding muscles, two pairs; 

 stomach long and straight, sustained by inflections of the visceral sheath ; 

 intestine convoluted dorsally, terminating between the mantle-lobes on the 

 right side; oral arms disposed in about six close whirls, their cavities opening 

 into the prolongation of the visceral sheath in front of the adductors. 



Observations on the living Lingula are much wanted; the oral arms 

 probably extended as far as the margins of the shell ; and the pedicle, which 

 is often nine inches long in preserved specimens, is doubtless much longer, 

 and contractile when alive. The shell is horny and flexible, and always of a 

 greenish colour. 



Distr. 1 sp. India, Philippines, Moluccas, Australia, Feejees, Sandwich 

 Ids. W. America. 



Fossil, 34 sp. L. Silurian . N. America, Europe, Thibet. 



Lingula 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. L. Davisii, L. Silurian, Tremadoc, has a pedicle- 

 groove like Obolus, fig. 168. (Salter). 



Fig. 168. Ventral valve. Fig. 169. Dorsal valve. 



Obolus Davidsoni (Salter). Wenlock limestone, Dudley. 



A, posterior adductors ; B, sliding muscles ; C, Anterior adductors. 



. The pedicle-scar in the centre of fig. 168 has no letter. 



OBOLUS, Eichwald. 



Syn. Ungula, Pander ; Aulonotreta, Kutorga. 



Etym. Obolus, a small Greek coin. Type, 0. Apolliuis, Eichw. 



Shell orbicular, calcario-corneous, depressed, sub-equivalve, smoot ; 

 hinge-margin thickened inside, and slightly grooved in the ventral valve , 

 posterior adductor impressions separate ; anterior pair sub-central ; impres- 

 sions of sliding-rnuscles lateral. Fig. 168, 169 (after Davidson.) 



Fossil, 4 sp. L, and U. Silurian. Sweden, Russia, England, U. States. 



CLASS V. CONCHIFERA, LAMARCK. 



(Lamelli-oranchiata, Blainville.) 

 The bivalve shell-fish, or Conchifera, are familiar to every one, under the 



