256 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



In thin sections the amalgamated cell walls and the very large 

 acanthopores composed of a clear, almost structureless tissue are the 

 most noteworthy features of the more minute details. Wherever an 

 acanthopore occurs the walls are greatly thickened, but they are quite 

 tliin in the intervening spaces. This alternate thickening and thin- 



FiG. 148.— Orbipora souda. a, side vte-w, natural size, of the solid, massive type-specimen; 6, 



TANGENTIAL SECTION, X20, SHOWING ZOCECIAL STRUCTURE; C, VERTICAL SECTION, X8, ILLUSTRATING 

 THE USUAL tabulation; d, ANOTHER VERTICAL SECTION, X20, "WITH DIAPHRAGMS MORE REGULARLY 

 PLACED IN THE MATURE ZONE. OrTHOCERAS LIMESTONE (B3), REVAL, ESTHONIA.. 



ning produces a very irregular aperture with the acanthopore some- 

 times projecting some distance into the cell cavity. Vertical sections 

 show an occasional diaphragm, although most of the tubes are with- 

 out them. The minute structure of the walls and acanthopores is 

 exactly the same as in Orhipora distincta. 



