BRYOZOA. 265 



Erldotrpya.] 



genus will include beside the following, probably two as yet undescribed species 

 from the Trenton of Kentucky, Batostomella simulatrix Ulrich, a widely distributed 

 species from the upper beds of the Hudson River group, Trematopora echinata Hall, 

 from the Niagara group of Indiana, T. ? (Trematella ?) corticosa Hall (see Pal. N. Y., 

 vol. vi, p. 15, pi. X, figs. 1-10), from the Lower Helderberg of New York, and Batosto- 

 mella obliqua Ulrich, from the Hamilton group of Michigan. 



The systematic position of the genus, though in a measure doubtful, is probably 

 intermediate between Homotrypa (compare H. similis Foord) of the Monticuliporidce, 

 and Bythopora, Miller and Dyer, of the Batostomellidce. Because of the absence of 

 cystiphragms it will be best to embrace the genus provisionally in the latter family. 



EKIDOTKYPA MUTABILIS, n. sp. 



PLATE XXVI. FIGS. 20-32. 



Zoarium ramose, dividing at rather long but irregular intervals ; branches 2 to 

 6 mm. in diameter, the younger examples slender and nearly cylindrical, the old 

 ones more or less irregular. Considerably over half of the hundreds of specimens 

 seen are from 3.5 to 4.5 mm. in diameter. Zooacial apertures variable, the changes 

 due chiefly to age, always oblique, the degree decreasing with age ; walls thick, 

 generally ridge-shaped and highest posteriorly, sloping gradually down into the 

 apertures. In young examples also in old ones on which a new layer of zooecial 

 tubes was formed the apertures may be exceedingly oblique and drawn out 

 anteriorly. With age they became gradually more direct. The arrangement of the 

 apertures is always more or less irregular, some of the short rows having six, others 

 seven, and occasionally eight in 2 mm. Small maculae, either pitted or irregularly 

 sculptured, commonly present in the older examples. In others the maculae are 

 represented by clusters of zooecia which, though a little larger than the average, are 

 distinguished from them chiefly by the greater thickness of the interspaces. The 

 mesopores too are most variable. In some cases, but this is rare, they will appear to 

 be wanting over large portions of the surface (see fig. 24); in others they may be 

 twice as numerous as the zooecia. As a rule, however, they are to be counted as 

 few, appearing at the surface, except in rare instances, only as occasional shallow 

 depressions between the zoo3cial apertures. True acanthopores probably wanting, 

 but small knots at the angles of junction may be noticed. 



Internal characters: These are, luckily, fairly constant in all essential features, 

 the principal variations observed being in the number of mesopores. Figs. 26 to 28 

 represent parts of sections prepared from an average example. Diaphragms occur 



