264 
DEVONIAN FAUNA. 
As far as can be judged from the very fragmentary specimens, it exactly 
agrees with the shell described as Euomphalus serpula by d' Archiac and de Verneuil 
(in part), which appears to be the same as Serpnlaria centrifuga, F. A. Romer. 
Clarke remarks that the apical whorls of this form are in contact. 
There does not seem any great difference between the American form described 
by Hall as Eu. laxus and Romer's shell, but it appears to expand more rapidly, 
to be more coarsely ornamented, and to have a slightly expanded mouth; it is 
therefore probably distinct. In Phanerotinus paradoxus, Winchell,^ the apical 
whorls are united and the base of the whorls more deeply convex. 
VIII. Family. — Trochid^, d'Orbigny, 1837. 
1. Sub-family. — TuooHiNiE, Swainson, 1840. 
1. Genus. — Plagiothyra, gen. nov. 
Shell conical, of few broad, rapidly increasing whorls, generally more or less 
nodulated or spirally ridged. Base convex or flattish. Mouth subcircular or 
subtriangular, obliquely situated, its upper angle being much in advance of the 
lower. Columella arched. Outer lip sharp. Inner lip flattened, diffuse, callous, 
bearing upon the centre of its inner side a large, prominent, transversely 
flattened tooth. Shell-structure massive. 
The two shells described below, though very different in general shape, appear 
to belong to the same type, so that they may be generically united. Their most 
prominent feature seems to be the possession of a single large median tooth on 
the inner lip of an obliquely receding mouth. Thus they clearly belong to the 
Trochidee, and come very close to the genus Monodonta, from which they are 
separated by the absence of denticulations in the outer lip, and the position and 
character of the tooth on the inner lip. The columella, also, is not truncated. 
The genus Turcica^ is distinguished from this by its very thin shell, and by the 
character of the teeth upon the columella ; and the genus Naticodon,^ by its less 
oblique aperture and its globose naticiform shell. 
The very great variety of the generic names given to PI. purpura, d'A. and 
de Vern., sp , by different authors shows the difficulty that has been experienced 
in the attempt to fix its proper position. As it does not seem possible satis- 
factorily to allocate either it or its fellow-species to any known genus, the only 
course open has been to found a new genus for their reception. 
' 1863, Winchell, ' Proc. Acad. Sci. Philad.,' p. 21. 
2 1858, Adams, ' Genera Eeceut Mollusca,' vol. i, p. 423. 
* 1847, Kyckholt, ' Melang. Pal.,' vol. i, p. 75. 
