Helopora.] ' 



BRYOZOA. 



193 



Helopoea mucronata TJlrich. 



PLATE III, FIG. 10. 



Helopora mucronata Ulbich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 192. 



Segments spine-like, slightly curved, 3.5 to 4.0 mm. long, tapering downward from 

 the truncate upper end, where the diameter is 0.6 or 0.7 mm., to the acute and finely 

 striated basal extremity. Zocecial apertures oblique, the inferior and lateral margins 

 elevated, arranged longitudiaally and spirally, with from six to eight forming a com- 

 plete volution, and six in about 2.5 mm. lengthwise. Above each zooecial aperture 

 two short striae. Obscure longitudinal ridges sometimes formed by the coalescence 

 and continuance of the elevated lateral margins of the zooecial apertures. 



The curved and tapering form, the acute lower extremity, and the very slight 

 development of the lorigitudinal ridges, distinguish this species from all the associ- 

 ated jointed Bryozoa, as well as from H. divaricata and H. spiniformis. None of the 

 other species of the genus are sufficiently near to require comparisons. 



Though readily distinguishable from all the known varieties of segments of the 

 associated Arthroclema armatum, it may yet be shown by complete zoaria of the latter 

 that H. mucronata is but another form of segment of Arthroclema. Until such evi- 

 dence is discovered we had best leave them as at present. 



Formation and locality. — Galena shales, near Cannon Falls, Minnesota, where it is associated with 

 Helopora quadrata, Arthroclema armatum, and several species of Nematopara. Also at St. Paul. 



Mus. Reg. No. 8112. 



Helopoea quadeata, tt. sp: 



" 'i 



Fig. 10, Helopora quadrata Ulbich. Galena shales, near Cannon Falls, Minnesota, a, hasal par 

 of a segment; 6, the central portion of another, and c, the upper extremity of a third, all x 18. 



Segments very slender, quadrate in cross-section, the angles sharp, the sides 

 each about 0.28 mm. wide; entire length unknown, none of the segments at hand 

 being complete, probably between 5 and 6 mm.; lower end bulbous, the upper with 

 two flattened articulating faces. Zooecial apertures in four longitudinal rows, one 

 on each of the concave sides ; ovate, a little oblique, the inferior and lateral margins 

 with a strong rim, produced backward, either straight or obliquely, as a sloping 

 ridge. Zooecial apertures separated by distances very nearly equal to their long 

 diameter, with nine or ten in 3 mm. 



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