BRYOZOA. 325 



Bythotrypu laxata.l 



type that became extinct or was reabsorbed into the parent stock, failing to estab- 

 lish a permanent line of development. In that case Crepipora? epidermata Ulrich, 

 from the Hudson River rocks of Illinois, would seem to be the earliest known point 

 in the direct line to Fistulipora. 



BYTHOTRYPA LAXATA Ulrich. 



PLATE XXVIII. FIGS. 21-25. 



Fistulipora? laxata ULRICH, 1889. Contri. Micro-Pal. Cambro-Sil. Eocks, Can., pt. ii, p. 37. 



Zoarium irregularly massive, usually beginning its growth upon some foreign 

 body, the exposed under side strongly wrinkled and covered with an epithecal mem- 

 brane. Specimens vary greatly in size, the smallest seen being about 12 mm. in 

 diameter and 5 mm. or less high, while' the largest is an oval mass 150 mm. long, 

 120 mm. wide, and about 70 mm. high. In the lower third of the Trenton shales 

 they are all small, none observed exceeding 50 mm. in diameter. In the middle 

 and upper thirds specimens between 75 and 100 mm. wide are not rare, but masses 

 exceeding that size have been met with only in the upper part of the Galena shales. 



Zooecial apertures subovate, nearly equal, direct or a little oblique, the lunarium 

 broad, sharply elevated, sometimes seeming to arch slightly over the aperture; their 

 arrangement appearing more irregular than it is, with nine or ten in 5 mm. Meso- 

 pores abundant, varying greatly in size, a few quite as large as the zooecia from 

 which they are distinguished by their more angular and irregular form and in being 

 without a lunarium. Mesopores forming larger or smaller clusters at irregular 

 intervals from which the zocecial apertures are turned in a radial manner. These 

 clusters are most inconspicuous even difficult to make out under the glass except 

 under certain conditions of weathering when they stand out as subsolid spots. 

 Under ordinary circumstances the whole surface seems to be occupied uniformly 

 by an irregular network of cells. 



Internal characters: In transverse sections the appearance, aside from an unusual 

 irregularity and looseness of arrangement, is much as in species of Fistulipora. The 

 zocecia are irregularly pyriform or ovate in cross section, and have thin walls. The 

 lunarium, though often not a very marked feature, is still always determinable by 

 the more regularly curved semi-circular form of the lunarial side of the circumfer- 

 ence of the zooacium, the opposite side being, if not angular, at any rate always 

 drawn to a circle of greater diameter than the lunarial side. Occasionally one or 

 both ends of the lunarium may project into the zooecial cavity (see fig. 25). Not 

 infrequently also the lunarial side is thickened by a light-colored deposit upon the 



