148, EOCENE AND LOWER OLIGOCENE CORAL FAUNAS. 



gitudinal section, perpendicular to septal plane, shows zigzag series of cal- 

 citicatiou centers, the zigzag arrangement corresponding to the undulations 

 of the septum. One of these rows of centers is 0.27 mm. long and contains 

 apparently four calcification centers. The next series measures about 0.18 

 mm. and contains three centers. The dissepiments are usually attached at 

 the elbows of the zigzags. The f^bro-crystals diverge upward awaj- from 

 the septal plane. In the longitudinal tangential section of another sep- 

 tum the middle point of one calcitication center measures 0.09 ram. from 

 the middle point of the center nearest it. 



Neither of the two thin sections that I had made has cut a septum par- 

 allel to its flat surface; therefore the structure in the septal plane must be 

 inferred from the two other sections. The structtire is the same as in 

 Ilaimesiastrmi covferta, except the calcification centers com^josing the tra- 

 becultB are larger than in that species (cf. ante, p. 144). 



The wall. — The wall is a pseudotheca formed by the fusion of the thick- 

 ened distal ends of the septa. A portion of a section across one corallite 

 shows a problematic line concentric with the calice between a few septa. 

 In this place there may be a few calcification centers between the distal ends 

 of the septa. Excepting this instance the wall is clearly a pseudotheca. 



The columella. — Tliis is fomied by the fusion of the inner ends of six or 

 more septa. The inner ends of the septa are thickened and usually fuse 

 solidly, making a firm, strong columella. 



lliis species is most astonishingly similar to Haimesiastrcea conferta. 

 The surface of Gabb's type material being eroded, sucli a criticiil compari- 

 son as is desired can not be made. The thi'ee following distinctions can be 

 pointed out: First, the septa of H. pctrosa are more thickened at the wall 

 than in H. conferta; second, the wall of the former is a pseudotheca, whereas 

 that of the latter species appears to be originally a true theca; third, the 

 calcification centers composing tlie septal trabeculse of H. petrosa are larger 

 than in H. conferta. Because of the difierence in the character of the wall, 

 some authorities would probably be inclined to place the two species in dif- 

 ferent genera. In my opinion the two forms belong in the same genus. 



The two species are necessarily difi'erentiated by microscopic charac- 

 ters, which I believe are good. When more abundant and more perfect 

 material of Gabb's species is found, we maj' lie able to discover gross char- 

 acters, upon which the difi'erentiation may be based. 



