190 



BULLETIN 17, UNITED STATES NATIONAL, MUSEUM. 



has given a good description of the American types which applies 

 equally well to the Russian specimens. His description, slightly 

 changed to agree with the present illustrations^ follows: 



Zoarium ramose, growth rather irregular; branches rounded, sometimes nodular 

 or lobate, and varying in diameter from 3 to 8 mm. ; surface generally without monti- 

 cules, and when these are present they are low and broad ; small macule or clusters 

 of mesopores are not infrequently present. Superficial characters of zooecia and 

 mesopores variable. In some, and these are in most cases well-preserved examples, 

 the zooecial apertures are irregular both in form and arrangement, with thin walls, 

 partly separated by mesopores numbering one or two to each zooecium. In these 



Fig. 101.— Homotrypellainstabilis. a and 6, two fragments, natural size; c, surface of awell- 



PRESERVED EXAMPLE, X18; d, TANGENTL&.L SECTION, X18, PREPARED FROM THE ORIGINAL OF a; 

 e, ANOTHER TANGENTIAL SECTION, X18, SHOWING PORTION OF A MACULA AND ZOCECIA WITH MORE 

 NUMEROUS ACANTHOPORES; /, SMALL PORTION OF A TANGENTIAL SECTION, X50, ILLUSTRATING THE 

 MINUTE STRUCTURE MORE CLEARLY; g, VERTICAL SECTION, X18, WITH TEE USUAL DISTEIBUTIOKr OF 

 CYSTIPHEAGMS AND DIAPHRAGMS. RHINIDICTYA BED OF THE BLACK RlVEB (DECOEAH) SHALES, ST. 



Paul and Minneapolis, Minnesota. (After Ulrich.) 



specimens the acanthopores are small yet prominent and sharp, and number from one 

 to three to each zooecium. The mesopores are always smaller than the zooecia, 

 but vary occasionally in shape, size, and arrangement. In many other examples 

 both the zocecia and mesopores are smaller and their walls correspondingly thick, 

 while the acanthopores are blunt and thicker. In most cases a little wearing suffices 

 to obscure the mouths of the mesopores, so that they are readily overlooked. Twelve 

 or thirteen of the zooecia occur in 3 mm. 



Internal characters. — Tangential sections of this species present an unusual variety 

 of appearances. In the majority of sections, providing they are not too deep, the walls 

 of the cells are very thick, with not a sign of cystiphragms in the zocecial cavities. 

 When a second or peripheral series of cystiphragms has been developed a very different 

 appearance is obtained. Now the walls are thinner, and a cystiphragm, leaving from 

 one-third to one-half of the zooecial cavity open, is to be seen in each of the zocecia. 

 In all cases the polygonal lines of contact between the two sets of cells is sharply de- 

 fined, and the walls of both approximately of equal thickness. The acanthopores 



