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, also at AH Creek. 



The Orbitoides are almost exactly the same as those figured 

 by Brady from Sumatra (Geol. Mag. 1875, PI. XIV., Figs. 2 and 

 3), and may for the present be called O. dispansa, Sow., though 

 I could not assert the specific distinctness of the specimens placed 

 under that name, and I should prefer to adhere to my former 

 treatment of the nomenclature as recorded in the Journal of the 

 Geological Society, 1892, p. 532. Silvestri has referred the Form 

 called O. dispansa to Lepidocyclina marginata Micheloti in a 

 Paper in Atti Acad., romana 1906. (See also Silvestri "Sulla Or- 

 bitoides gumbelii" in Atti Acad, romana Dec. 1905, page 34). 

 The superficial characters of our Form are similar to those illus- 

 trated by Silvestri. Among the Marbela specimens we ma}^ 

 roughl}*- distinguish three Forms ; 1 .° the small or young forms, 

 nearly 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 growth the extension of the 

 sarcode body 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 power 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 



