302 THE PALEONTOLOGY OF MINNESOTA. 



_[Stromatotrypa ovata. 



Apertures oval, separated by depressed interspaces, the peristomes minutely papil- 

 lose. Mesopores abundant, beginning on the basal lamina, decreasing in size with 

 age, closely tabulated, the diaphragms finely punctured ; mouths rarely visible, 

 closed by a common dermal sheet. True acanthopores wanting. 



Type : Stromatotrypa ovata, n. sp. 



The species upon which this genus is based has much to remind us of. the 

 Devonian Pinacotrypa, Ulrich. But as that genus is clearly a branch of the Fistuli- 

 poridce, and Stromatotrypa most likely not far removed from Batostoma and Tremato- 

 pora, it is evident that there can be no real affinity with Pinacotrypa. The 

 arrangement with the Diplotrypidce, though provisional, is strongly suggested by 

 the dimorphic character of the zoarium and the supposed relationship to Bastos- 

 toma. 



Stromatotrypa ovata, n. sp. 



PLATE XXIV, FIGS. 24-31. 



Zoarium consisting of one or more exceedingly thin layers, varying in thickness 

 from 0.5 to 2.0 mm., occasionally attached to shells, but more commonly encrusting 

 other Bryozoa. Sometimes the attachment is incomplete, and in a few cases the 

 zoarium is free, with the underside wrinkled. Surface without monticules, but 

 exhibiting at irregular intervals smooth spots of greater or less extent on which 

 the zooecia are generally farther apart than usual and the apertures of some of them 

 closed. Zooecial apertures oval, peristomes thin, in contact at limited points or more 

 or less widely separated, the arrangement inclining to be irregular, with six to eight, 

 commonly seven, in 3 mm.; length of apertures varying from 0.3 to 0.45 mm., the 

 average about 0.28 by 0.38 mm. Interspaces depressed, of variable width, apparently 

 smooth in the youngest stages, minutely granulose in the aged conditions. Small 

 acanthopore-like elevations generally where peristomes come into contact. The 

 best preserved specimens exhibit also a row of minute papillae crowning the peri- 

 stomes. 



Internal characters: In tangential sections the zooecial walls are thin and in 

 contact at one or more points, or they may be completely isolated by mesopores 

 varying considerably in shape and size. Attached to the walls, or more frequently 

 occupying the angles of junction, are minute rounded and thin-walled cells that are 

 perhaps to be regarded as peculiarly modified acanthopores. Good sections show 

 besides a large number of smaller dots in the interspaces and zocecial walls. In 

 vertical sections both sets of tubes rise abruptly from the basal lamina, and the 

 mesopores decrease more or less conspicuously in size in nearing the surface. 



