BRYOZOA. 377 



marked divisional line between the walls of adjoining cells. 

 Mesopores practically absent, but small abortive cells some- 

 times occur among the large zooecia forming the monticules. 

 Acanthopores sometimes absent, generally abundant. 



AMPLEXOPORA Ulrich. Zoaria ramose. Zooecia polygonal, 

 forming prismatic tubes, which are thin-walled in the axial 

 region and more or less strongly thickened in the peripheral. 

 Mesopores wanting. Acanthopores always present, varying in 

 size and number. Diaphragms complete, horizontal. 



Type: A. dngulata Ulrich. Range, Trenton to Hamilton. 



MOXOTRYPELLA Ulrich. In most respects like Amplexopora, 

 but differing in the absence of acanthopores. In one section of 

 the genus, the zooecia are often separated from each other in the 

 peripheral region by closely tabulated interspaces, that simu- 

 late mesopores. 



Type: M. sequalis Ulrich. Range, Trenton to Hamilton. 



PETALOTRYPA n. gen. Zoaria bifoliate, consisting of irregular, 

 compressed branches or simple fronds, celluliferous on both 

 sides. Zooecial tubes prismatic, arising from a strongly flexu- 

 ous mesial line. Apertures subcircular or polygonal. Mesopore- 

 like interspaces that do not differ in their tabulation from the 

 zooecia may occur. Very small acanthopores (?) occupy many 

 of the angles of junction. 



Type: P. compressa n. sp. Range, Lower Helderberg to Ham- 

 ilton. 



ATACTOPORA Ulrich. Zoaria thin, incrusting, often composed 

 of more than one layer. Zooecia with moderately thick walls, 

 the apertures indented or floriform, a peculiarity due to the 

 position of the very abundant acanthopores. These always 

 encroach upon the visceral cavity. Rather large solid maculae 

 or monticules, composed of numerous abortive cells, completely 

 filled by calcareous deposit, stud the surface at rythmical inter- 

 vals. Diaphragms thin, few, sometimes wanting. 



Type: A. Im-suta Ulrich. Range, Trenton? and Cincinnati. 



LEPTOTRYPA Ulrich. Zoaria varying from thin incrustations 

 to free forms of discoidal, spiral, or elongate conical shape; 

 irregularly massive species occur also. Zooecia polygonal, with 



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