238 



MANUAL OF THE MOLLUSCA. 



lides (e. g. the sea-mouse, Aphrodite], The relation of the animal to the 

 perforate and imperforate valves is shown to be the same as in Terebratula, 

 by the labial fringe ; but the only process which can possibly have afforded 

 support to the oral arms, is developed from the centre of the ventral valve, 

 as in Crania. Baron Ryckholt has represented a Devonian fossil from Bel- 

 gium, with a fringed border; but if this shell is the Crania obsoleta of 

 Goldfuss, the fringe must belong to the shell, and not to the mantle. 



Distr. 7 sp. W. Africa, Malacca, Peru, Panama. 



Fossil, 29 sp. Silurian . Europe, U. States, Falkland Ids. The (27) 

 Palaeozoic and secondary species constitute the genus Orbiculoidea, D'Orb. 

 (Schizotreta, Kutorga.) In some species the valves are equally convex, 

 and the foramen occupies the end of a narrow groove. 



Sub-genus, Trematis, Sharpe. (= Orbicella, D'Orb.) T. terminalis, 

 Emmons. Valves convex, superficially punctate ; dorsal valve with a thick- 

 ened hinge-margin (and three diverging plates, indicated on casts ; Sharpe.} 

 Fossil, 14 sp. L. and U. Silurian. N. America, Europe. 



Fig. 163. Fig. 164. Exterior. 



Fig. 163, a, Interior, 



SIPHONOTRETA, Vemeuil. 



Etym. Siphon a tube, tretos perforated. 



Types, S. unguiculata, Eichw. fig. 163, ]63, a. S. verrueosa, fig. 164. 



Shell oval, bi-convex, slightly beaked, conspicuously punctate, or spiny ; 

 beak perforated by a tubular foramen; hinge-margins thickened; ventral 

 valve with four close adductor scars surrounding the foramen. The spines 

 are tubular, and open into the interior of the shell by prominent orifices. 

 ( Carpenter.} S. anglica, Morris, has moniliform spines. 



Fossil, 6 sp. L. and U. Silurian. Brit. Bohemia, Russia. 



? Acrotreta (sub-conica) Kutorga, L. Silurian, Russia. Shaped like 

 Cyrtia, with an apical foramen ; no hinge. 



FAMILY VIII. LINGULIDJS. 



Shell oblong or orbicular, sub-equivalve, attached by a pedicle passing out 

 between the valves ; texture horny, minutely tubular. 



