. Bulletin - United States National Museum. Science. 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
. Bulletin - United States National Museum. Science. 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.—Tan
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