STROPHOMENIDA. 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. planumbona and euglypha it becomes convex ; 
these distinctions are not even subgeneric. 
sTROPHODONTA, Hall, 1850. Shell with a denticulated hinge- 
line. Silur.; N. Am. S. prisca, Hall. 
LEPTANA, Dalman, 1827, emend. Davidson. (Plectambonites, 
Pander [part], 1830. Leptznulopsis, Haupt.) Valves regularly 
curved ; dorsal concave, thickened, muscular impressions elon- 
gated. Fossil, 41 sp. Lower Silurian—Lias; N. Am. and Eur. 
The Lias Leptznas resemble Thecidia internally ; they are free 
shells, with sometimes a minute foramen at the apex of the 
triangular deltidium. 8S. transversalis, Dalm. (exxxviii, 53; 
CXXXix, 54, 55). 
LEPTAGONIA, M’Coy, 1844. (Plectambonites, Pander [part], 
1830. Leptena, King, 1846. Strophomena, Meek, 1873.) Silu- 
rian—Carboniferous. 8S. rhomboidalis, Dalm. (cxxxviii, 51, 52). 
DavipsontA, Bouchard, 1849. 
Htym.—Dedicated to the author of the Monograph of British 
Fossil Brachiopoda. 
Distr.—Fossil, 3 sp. Devonian. D. Verneuili, Bouchard 
(exxxix, 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 exisfence 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. 
AmpuHicuina, Laube, 1865. 
Etym.—Amphi, about, and clino, a slope. 
Distr.—2 sp. . Triassic; St. Cassian, Austria. A. dubia, 
Munster. 
Shell inequivalve, circular, excavated, smooth; ventral valve 
convex, beak short; perforated ; dorsal concave ; hinge-line very 
short and suboblique; area wanting; deltidium triangular, 
5) = 
distinct; structure of the test fibrous, squamose; externally 
