24.0 CRETACEOUS GASTROPODA 
8. Bivonia, Gray, 1850. Mérch (Proc. Zool. Soc., Lond., 1862, p. 54). 
The shells, if not perfect with the margins of the aperture, are very difficultly 
distinguished from Spiroglyphus. 
9. Thylacodes, Guettard, 1774. (Mérch, Proc. Zool. Soc., Lond., 1862, 
p. 64). 
Testa plerumque solitaria, repens, quandoque spirata, sepe liris 3-5, longitudina- 
libus, nodulosis ornata ; apertura circularis, nunquam contracta. Testa nuper nata 
bulimoidea, apertura antice subeffusa (Morch). 
This genus is largely represented among living shells; it mostly contains the 
irregularly coiled species, although they are generally spiral, when young. From 
the tertiary beds several are known. The greater number of Serpulorbis, lately 
described by Deshayes from the Paris basin, belong to Pigeons in the sense in 
which the genus has been determined by Mérch. 
A very characteristic species has been procured from the Arrialoor beds near 
Comarapolliam, 7h. lamellosus, n. sp. There are besides several fragmentary speci- 
mens of species, belonging to this genus, from the Ootatoor beds near Odium and 
Moraviatoor, but they must for the present be left undetermined, until better 
materials can be obtained. 
L. TUBULOSTIUM, Stoliczka, 1868, (vide p. 237). 
1.—TUBULOSTIUM DISCOIDEUM, Stoliczka, Pl. XVIII, Figs. 20-25. 
Tub. testa discoidea, planorbulari, anfractibus quinis sew senis, angulatis ad 
peripheriam obtuse convexis, supra atque infra, et ad marginem suturalem subcarinatis, 
callositate tenui junctis, sublevigatis; striis incrementi in dorso convexe curvatis, inter- 
dumque nonnullis sulcis spiralibus subobsoletis interruptis; ultimo anfractu prope 
rostrum callositate magna instructo ; rostro valde contracto atque prolongato. 
The diameter of our largest specimens does not exceed 14 mm., and the height of the last 
volution 2 mm. 
This little discoid shell rather more resembles a Serpula than a Gastropod, but 
the three layers of which it is composed, are distinctly traceable, though they are 
usually very thin. The embryonal whorls, although very seldom preserved, are 
also distinctly spiral, and the succeeding whorls are coiled in a regular symmetrical 
plane. They are nearly quadrangular in the outer section, each being attached to 
the back of the previous one by a thin callose layer, carinate above and below. 
The last whorl is slightly convex on the outer periphery. The striw of growth 
are very fine, so as to be sometimes scarcely traceable. Beside these there occa- 
sionally appear on the surface of the shell a few spiral furrows, which are some- 
what more distinct on the outer periphery. The last whorl is thickened towards 
aperture, which contracts suddenly, and extends into a more or less pro- 
longed tube. 
The only species, which shows a great relation to our Indian fossil, is the 
Vermetus tumidus, Sowerby (Min. Conch., Vol. VI, p. 195, pl. 596, fig. 4,) from the 
