BRACHIOPODA. 



229 



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

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

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

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

 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. ORTHID^.* 



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

 wide 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 (f) ; 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 (0). 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 



ORTHIS, Dalman. 



Etym. Orthos, straight. Type, 0. rustica, PI. XV. fig. 23. 

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

 bonites (inflexa) Pander. Orthambonites (calligramma) Pander. 



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

 stituting ides for the last syllable of the genitive case. 



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

 casts of this fossil were called hysterolites by old authors. 



