288 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



crenulated in the mature region where the tliickness is again increased 

 by the deposition of solid tissue along their sides. Here diaphragms 

 are more numerous and nearly always appear to be complete. In the 

 mesopores diaphragms occur at intervals of one-half their diameter 

 apart. Frequently the mesopores become filled with soHd tissue to 

 such an extent that they are indistinguishable in sections and form 

 solid interspaces at the surface. 



^ 



MMmK 



Pig. 175.— Hemipheagma glabeum. a and &, two fragments, natubal size; c, basal portion of a 



ZOAEIUM, NATXmAL SIZE; d, TANGENTIAL SECTION, X20, EXHIBITING THICK-WALLED, ANGULAR ZOCECIA 

 AND mesopores; e, vertical SECTION, X20, SHOWING FEWER SEMIDIAPHRAGMS THAN USUAL;/, SEV- 

 ERAL ZOCECLi. OF A TANGENTIAL SECTION, X35, ILLUSTRATING WALL STKUCTtTRE. WaSSALEM BEDS (D3) 

 UXNORM, ESTHONIA. 



The internal structure of numerous specimens of this well-marked 

 species has been studied and the usual characters are illustrated in 

 figure 175. In figure 175 e the semidiaphragms are not as numerous 

 as in other vertical sections while the complete diaphragms are more 

 abundant. It is probable that most of the latter are in reality incom- 

 plete and only appear complete because of the way they are cut by 

 the section. 



Externally the slender, branching, smooth stems with thick-walled 

 zooecia and mesopores, and internally the strongly crenulated walls 

 of the immature region, and the occurrence of semidiaphragms are 

 characters which will aid in separating the present species from asso- 



