6 BULLETIN 103/ UNITED STATES NATIONAL MUSEUM. 



mens after decalcification, though the outlines of the cells may be 

 recognized here and there. As microtome sections of the decalcified 

 fossil material are out of the question, comparisons of structure of 

 the recent and fossil must naturally be based upon calcareous ground 

 sections. And in comparing the cell structure in sections of the re- 

 cent decalcified specimens (pi. 3) with that shown in ground sections 

 of the calcareous fossils, it is necessary, of course, to bear in mind 

 that cells in calcareous ground sections of the Corallinaceae com- 

 monly appear much more rectangular than in decalcified sections of 

 the same material. 1 In the sections of the fossil material thus far 

 made there are no certainly recognizable traces of sporangial cavi- 

 ties, but this is true in almost an equal degree of calcareous 

 ground sections of the recent specimens except as to the surface of 

 the plant (fig. 1, pi. 2), where the sori are, in fact, so decidedly 

 superficial or even exserted that they could, perhaps, hardly be 

 expected to persist in the fossil state. 



In the same locality with the type- specimens (Howe 6832) there 

 occurs an outwardly somewhat similar plant (Howe 6837) that we at 

 first suspected to be the antheridial form of A. episporum,, but certain 

 recognizable, though possibly unimportant, differences in the form, 

 size, and zonation of the perithallic cells have restrained us from so 

 considering it. The antheridial conceptacle (cavities) in this 6837 

 are 64-95 tx broad and 60-72 [x high ; they become copiously embedded 

 by the continued upward or outward growth of the thallus. 



LITHOTHAMNIUM - VAUGHANII, new species. 



Plate 7, iigs. 1 avid 2, and plate 8. 



Thallus forming at first expanded crusts 1-2 mm. thick, these be- 

 coming overgrown, irregularly stratified, and 10 mm. or more thick, 

 developing finally numerous, rather coarse, crowded anastomosing 

 branches, and forming masses 2-4 cm. or more high ; branches mostly 

 3-12 mm. in diameter, usually much flattened, occasionally subterete, 

 often reduced to anastomosing ridges, or sometimes appearing as 

 dome-shaped elevations 2 cm. or more broad; primary hypothallia 

 somewhat reduced, their cells 14-33 \x by 8-14 ix, rather irregularly 

 arranged (i. e., not distinctly "coaxial"), cells of medullary hypo- 

 thallia mostly 15-30 [x by 5-13 \>., secondary hypothallia numerous 

 and thin ; branches showing in section numerous narrow irregularly 

 flexuous, often subelliptic-lenticular or subcrescentic zones caused by 



1 For illustrations of these differences, see Lemoine, Ann. Inst. Ocganog., vol. 2, pt. 2, 

 p. 45, figs. 19-21, 1911. 



2 The writer believes, with Mme. Paul Lemoine, that the current rules of nomenclature 

 require that Philippi's original spelling of this generic name should be respected, even 

 though prevailing usage has modified the final syllable. Whether the rules of nomencla- 

 ture justify the use of this generic name for any of the species now bearing it is a more 

 complicated question. 



