192 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



phragms have enough tissue added to them to complete the structure. 

 A tangential section of a young example is shown in figure 102 d. 



The ramose form of growth, numerous tabulated mesopores, and 

 zooecia hned with cystiphragms, are characters different from all 

 other Russian forms save Eomotrypella crihrosa. The latter has a 

 more regularly inosculating form of growth, smaller branches, a short 

 mature region, and oblique aperture. 



Occurrence. — Abundant in the Rhinidictya bed of the Black River 

 (Decorah) shales at St. Paul, MinneapoHs, and other locahties in 

 Minnesota. Common in the Kuckers shale (C2), Baron Toll's estate, 

 and at Reval, and in the Echinospherites limestone (Cl), 4 miles 

 east of Reval, Esthonia. 



Plesiotype.—Csit. No. 57275, U.S.N.M. 



British Museum, specimens and thin sections from the Kuckers, 

 Baron Toll's estate. 



H^MOTRYPELLA CRIBROSA, new species. 

 Plate 13, text fig. 103. 



Zoarium growing from a thick, small basal expansion into narrow, 

 compressed, rounded branches that inosculate at intervals of 10 mm. 

 or less until there results a broad expansion usually spread in a plane. 

 Branches 2 to 4 mm. in diameter, surface smooth but marked at 

 regular intervals by distinct sohd clusters of mesopores. Zooecial 

 apertures obhque and long drawn out in young stages, direct, 

 polygonal, and thick-walled in old portions of the zoarium; seven 

 to eight zooecia in 2 mm. Mesopores most abundant in the maculae 

 but comparatively few elsewhere and usually closed at the surface. 

 Well-defined acanthopores are also few but numerous granules sim- 

 ulating these structures are present in the most mature regions. 



Vertical sections of this species are especially interesting. In the 

 immature zone the zooecial tubes are noticeably constricted where- 

 ever a diaphragm is inserted. In the outer portion of this zone large 

 cystiphragms occur and continue until the mature region is reached, 

 where the zooecial walls thicken, mesopores are introduced, and the 

 cystiphragms decrease in size but are in a crowded series. The dia- 

 phragms and cystiphragms of the immature zone average the distance 

 of a tube diameter from each other. Diaphragms are sparsely devel- 

 oped in the mature zone but here three to four cystiphragms occur 

 in the width of a zooecium. Other features of vertical sections are 

 the gradual bend from the immature to the mature zone, the narrow- 

 ness of the latter, and the thickening of the walls and closing of the 

 mesopores by layers of tissue as the surface is approached. 



Tangential sections taken close to the surface of mature specimens 

 show polygonal thick-walled zooecia with an occasional closed inter- 

 space. True acanthopores are occasionally noted, but smaller gran- 



