BEACniOPOl^A. 



379 



directed towards the centre of the valve ; teeth and impressions 

 like Rhynchondla. 



The shells of this genus differ from. RhynchoneUa chiefly in 

 the calcification of the oral suj^ports, a character of uncertain 

 value. 



Fossil, 21 species. Lower Silurian — Trias. America (Wel- 

 lington Channel! Falkland Islands), Eui^ope, Thibet. 



Anoplotheca lamellosa, F. Sandberger, 18d6, Devonian, Ehine, 

 is a species of Atrypa. 



Family IV. — Orthld^.* 



Shell transversely oblong, depressed, rarely foraminated ; 

 hinge-line wide and straight ; beaks inconspicuous ; valves 

 plano-convex, or concavo-convex, each with a hingo-area {h) 

 notched in the centre ; ventral valve with prominent teeth [t] ; 

 muscular impressions occupying a saucer-shaped cavity with a 

 raised margin ; adductor (a) central ; cardinal and pedicle 

 impressions (r) conjoined, lateral, fan-like; dorsal valve with a 

 tooth-like cardinal process between two curved brachial pro- 

 cesses (c) ; adductor impression (a) quadruple ; vascular impres- 

 sions consisting of six principal trunks in the dorsal valve, two 

 in the ventral, the external branches turned outwards and back- 

 wards inclosing wide ovarian spaces (o). Indications have been 



Dorsu.1 valve, t 



Fig. 182. Orthis striatida. 



Ventral valve. 

 Devonian, Eifel. 



observed, in several genera, of horizontally-coiled spiral arms ; 

 the space between the valves is often very small. The shell- 

 structure is punctate, except in a few instances, where the 

 original texture is probably obliterated. 



* The names of the families are formed from those of the typical genera, by sub 

 etituting ida for the last syllable of the genitive case. 



t From a specimen presented by M. De Koninck to the Britioh Museum ; internal 

 casts of this fossil were called hys. erolites by old authors. 



