BRACHIOPODA. "243 



The gradual development of the characters of Scenidium can be traced from 

 the typical features of Orthis, and, after attaining its extreme manifestation in 

 the Devonian, this combination abruptly disappeared, and no further repre- 

 sentatives in this line have been discovered. 



The incurved plate in the rostrum of the pedicle-valve, which in the orig- 

 inal diagnosis was regarded as a deltidium, appears to be due to the union of 

 the dental lamellae, forming a spondylium analogous to that in Clitambonites. 



The shell-substance of Scenidium is very finely fibrous, and from sections of 

 S. pyramidale appears to be impunctate. Mr. Davidson, however, has repre- 

 sented* an enlargement of the surface of S. areola, which shows superficial 

 granulations or pits which may be evidence of punctate structure. Accord- 

 ing to present knowledge Scenidium appeared in the fauna of the Trenton 

 group, and has had a wide distribution, disappearing in the Devonian. 



Besides the American species already mentioned, Mr. Walcott has described 

 S. devonicum, from the Lower Devonian horizon of the Eureka District, Nevada. 

 Scenidium Lewisi, Davidson, occurs in the Wenlock of England, Scotland and 

 the Isle of Gotland; S. Lewisi, var. Woodlandense, Davidson, in the Middle 

 Llandovery of Ayrshire ; S. Lewisi, var. Hughesi, Davidson, in the Coniston 

 grits; S. Shallockense, Davidson, and >S'. Grai/im, Davidson, in the Middle 

 Caradoc of Girvan.f Mr. Davidson has also referred to this genus the Orthis 

 Deshayesi, Bouchard, occurring in the Devonian beds of Ferques, Brittany ;:|: 

 and ffiHLERT describes Scenidiu?n Baylii (= Orthis Baylii, llouault), from the 

 Devonian of Gahard, in the west of France. § Two other Devonian species 

 have been described by Tschernyschew,|| from the Stringocephalus-beds of the 

 Urals, viz., S. Malleri and S. Uralicum. 



* British Devonian Brachiopoda. Siippl., p!. iii, fig-. 11 (!. 

 t See Davidson, British Silui-ian Urai'hiopi.rla. Suppl., i)p. 173-175. 188:?. 



J See Davidson, British Devonian Bracliiopoda. Suppl., p. ,")0. 1884 ; ami RKiAUX, Bull, ile la Societe 

 Academique de Boulogne, vol, i. 1872. 



§ Brachiopodes du Devonien de roue.st de la France, p. 4. 1.SS7. 



I Die Fauna des mittlereii und obereii Devon am West-Abhanjje lies Urals, pp. lOtJ, 1U7. 



