136 
Bulletin 35 
284 
trated from adjoining beds. This rock contained a considerable 
number of Orbitoides, similar to those found in the lowest beds 
of the Naparima Tertiaries. They show that the Orbitoides-bed 
underlies this series of strata and that the base of the tertiaries 
has been reached. This bed appears again near the San Fernan- 
do Railway' Station and at Point Bontour, al.so at Ali Creek. 
The Orbitoides are almost exactly the same as those figured 
b}' Bradi' from Sumatra (Geol. Mag. 1875, PI. XIV., Figs. 2 and 
3), and may for the present be called O. dispa?isa, Sow., though 
I could not assert the specific distinctness of the specimens placed 
under that name, and I should prefer to adhere to mj^ former 
treatment of the nomenclature as recorded in the Journal of the 
Geological Societ}’, 1892, p. 532. Silvestri has referred the Form 
called O. dispansa to Lepidocyclina marginata Micheloti in a 
Paper in Atti Acad., romana 1906. also Silvestri “Sulla Or- 
bitoides gumbelii’’ in Atti xVcad. romana Dec. 1905, page 34). 
The superficial characters of our Form are similar to those illus- 
trated by Silvestri. Among the Marbela specimens we may 
roughl}" distinguish three Forms ; i .° the small or young forms, 
nearl}' smooth ; 2.° the medium-sized forms, having a pitted or 
foveate surface ; these are much thinner towards the edges than 
the third form in which the superficial cells are converted into 
tubercles owing to the development of exogeneous deposit. This 
which may be called the adult form is almost evenly biconvex. 
In the small form the process of shell construction is going on, 
the sarcode body forming cells on a definite c\'clical plan for its 
reception. Having attained its full growdh the extension of the 
sarcode bod\’ is lodged in cells constructed for its accommodation 
on the outer surface of the disk, forming the pitted or foveolate 
structure of that surface. The organism growing older but still 
retaining some of its pow’er of secreting calcareous matter gradu- 
ally covers over or fills up these superficial cells with exogeneous 
deposit forming the lumps or tubercles seen in old specimens. 
The Orbitoides found at Point Bontour where I originally 
discovered the Orbitoides-bed are easily cut across whereby the 
internal arrangement of the chambers is seen. But the Marbela 
