STROPHOMENID^. 331 



approach their full growth, they then bend abruptly to one side ; 

 the dorsal valve becomes concave in S. alternata and rhomboid- 

 alis, whilst in S. idanumhona and euglypha it becomes convex ; 

 these distinctions are not even subgeneric. 



STROPHODONTA, Hall, 1850. Shell with a denticulated hinge- 

 line. Silur. ; N. Am. S. p7'isca, Hall. 



LEPT^NA, Dalman, 1827, emend. Davidson. (Plectambonites, 

 Pander [part], 1830. Leptiienulopsis, Haupt.) Valves regularly 

 curved ; dorsal concave, thickened, muscular impressions elon- 

 gated. Fossil, 41 sp. Lower Silurian — Lias ; N. Am. and Bur. 

 The Lias Leptsenas resemble Thecidia internally ; they are free 

 shells, with sometimes a minute foramen at the apex of the 

 triangular deltidium. S. transversalis, Dalm. (cxxxviii, 53 ; 

 cxxxix, 54, 55). 



LEPTAGONiA, M'Coy, 1844. (Plectambonites, Pander [part], 

 1830. LeptEena, King, 1846. Strophomena, Meek, 1873.) Silu- 

 rian — Carboniferous. S. rhomboidalis, Dalm. (cxxxviii, 51, 52). 



Davidsonia, Bouchard, 1849. 



Etym. — Dedicated to the author of the Monograph of British 

 Fossil Brachiopoda. 



Dist7\ — Fossil, 3 sp. Devonian. D. Verneuili, Bouchard 

 (cxxxix, 56, 57). Devonian; Eifel. 



Shell solid, attached by outer surface of the ventral valve to 

 rocks, shells, and corals ; valves plain, articulated ; ventral 

 valve with a wide area ; foramen angular, covered by a convex 

 deltidium ; disk occupied by two conical elevations, obscurely 

 grooved by a spiral furrow of 5-6 volutions ; dorsal valve with 

 two shallow lateral cavities ; vascular impressions consisting of 

 two principal submarginal trunks, in each valve, with diverging 

 branches ; cardinal and adductor impressions distinct. The fur- 

 rowed cones undoubtedly indicate the existence of spiral arms, 

 similar to those of Atrypa, but destitute of calcified supports. 

 The upper valve sometimes exhibits markings derived from the 

 surface on which the shell has grown. The mantle-lobes seem to 

 have continued depositing shell until the internal cavity was 

 reduced to the smallest possible limit. 



Amphiclina, Laube, 1865. 



Etym. — Amphi, about, and clino, a slope. 



Distr. — 2 sp. Triassic ; St. Cassian, Austria. A. dubia, 

 Miinster. 



Shell inequivalve, circular, excavated, smooth ; ventral valve 

 convex, beak short ; perforated ; dorsal concave ; hinge-line very 

 short and suboblique ; area wanting ; deltidium triangular, 

 distinct; structure of the test fibrous, squamose; externally 



