BRACHIOPODA. 



229 



small, often closely incurved: foramen round, sometimes completed by a 

 deltidium, often concealed : dorsal valve with a divided hinge-plate, supports 

 ing two broad spirally coiled lamellae ; spires vertical, closely appressed, and 

 directed towards the centre of the valve ; teeth and impressions like Bhyn- 

 chonella. 



The shells of this genus differ from Rhynchonella chiefly in the calcifi- 

 cation of the oral supports, a character of uncertain value. 



Fossil, 15 sp. L. Silurian — Trias. America (Wellington Channel ! 

 Falkland Ids.), Europe, Thibet. 



FAMILY IV. OuTHiD.E.* 



Shell transversely oblong, depressed, rarely foraminated; hinge-line 

 ^vide and straight; beaks inconspicuous; valves plano-convex, or concavo- 

 convex, each with a hinge-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-pro- 

 cess between two curved brachial processes {c) ; adductor impression {a) 

 quadruple : vascular impressions consisting of six principal trunks in the 

 dorsal valve, two in the ventral, the external branches turned outwards and 

 backwards inclosing wide ovarian spaces {o). Indications have been 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. 



Dorsal valve. ^ Ventral valve. 



Fig. 147. Orthis, striatula. Devonian, Eifel. 



Oethis, Dalman. 

 Orthos, straight. Type, O. rustica, PI. XV. fig. 23. 

 Syn. Dicoelosia (biloba) King. Platystrophia (biforata) King. Gonam- 

 bonites (inflexa) Pander. Orthambonites (caUigramma) Pander, 



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

 stituting idee for the last syllable of the genitive case. 



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

 casts of this fossil were called hysterolites by old authors. 



