8 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



haps most resembles Lithophyttum daedaleum Foslie and Howe 1 as 

 to general habit, but differs from it much in structure. 



In the best section, No. 35299 U.S.N.M., the one from which the 

 photographs (fig. 2, pi. 7 and pi. 8) were made, the coarse inter- 

 sporangial sterile tissue of the tetrasporic conceptacles is scarcely 

 shown, yet- the roofs of the conceptacles show unmistakable canals 

 and none of the conceptacles in section exhibits a single orifice, so 

 that we consider ourselves justified in inferring that the specimen in 

 question is tetrasporic and that it belongs in the genus Lithotham- 

 nium in the sense in which that name is currently applied to living 

 plants. In a section from another specimen under the same collec- 

 tion number, traces of the sporangia and of the intersporangial 

 sterile tissue are evident. It is to be observed also that the zonate 

 arrangement of tissues, as observed in a section, is essentially of the 

 character assumed by Mme. Lemoine 2 as being peculiar to the genus 

 Lithothamnium. The rather distinctly specialized nature of the 

 cells of the conceptacle roof is evidently a character of importance, 

 in whieh respect it differs markedly from the plant we are de- 

 scribing as Lithothamnium isthmi, as also in the distinctly zonate 

 structure of the thallus, the reduced hypothallium, the larger tetra- 

 sporic conceptacles, larger perithallic cells, etc. 



Among the more fully described fossil Lithothamnieae, L. vaugh- 

 anii may perhaps be compared with Lithothamnium suganum Roth- 

 pletz 3 from the Tertiary (" Scio-Schichten") of Val Sugana, near 

 Borgo in the Austrian Tyrol, but the conceptacles of the Panamanian 

 fossil are much larger (500-740 pi wide and 130-230 pi high vs. 250 pi 

 wide and 100 pi high) and the perithallic cells appear to average con- 

 siderably larger, being sometimes 13-22 pi high, while those of L. 

 suganum are described as 9-12 pi long. 



LITHOTHAMNIUM ISTHMI. new species. 



Plate 7, fig. 3; plates 9, 10, and 11. 



Thallus forming at first stratified crusts 3-12 mm. thick, but at 

 length developing tortuous anastomosing branches and forming large 

 rather solid, concrescent, f ruticose masses ; branches mostly 2-12 mm. 

 in diameter, much flattened or subterete, often subconic-cylindric,, 

 flexed-digitiform, or molariform; hypothallia showing regular con- 

 centric layers of cells (" coaxial ") ; hypothallium of the crustaceous 

 parts 160-480 pi thick, its cells 17-28 pi by 8-13 pi, transition to the 



1 Bull. N. Y. Bot. Gard., vol. 4, p. 133, pis. 83, 84, 93, 1906. 



■ Lemoine, Mme. Paul. Structure anatomique des Melobesiees. Application a la classi- 

 fication. Ann. Inst. Oceanog., vol. 2, pt. 2, pp. 27, 28, 1911. 

 "Zeits. Deuts. Geol. Ges., vol. 43, p. 319, pi. 17, fig. 4, 1891. 



