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. Grayice, 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 CEhlert describes Scenidium Baylii (= Orthis Baylii, Rouault), 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. Maileri and S. Uralicum. 
* British Devonian Brachiopoda. Suppl., pi. iii, fig. 11 c. 
t See Davidson, British Silurian Brachiopoda. Suppl., pp. 173-175. 1883. 
+ See Davidson, British Devonian Brachiopoda. Suppl., p. 50. 1884; and Rigaux, Bull, de la Societe 
Academique de Boulogne, vol, i. 1872. 
§ Brachiopodes du Devonien de l’ouest de la France, p. 4. 1887. 
| Die Fauna des mittleren und oberen Devon am West-Abhange des Urals, pp. 106, 107. 
