ART. 2] CYCLOSTOMATOUS BRYOZOA CANU AND BASSLER 87 



Genotype. — SipJiodidyum gracile Lonsdale, 1849. Cretaceous (Ap- 

 tian, Albian and Campanian). 



Affinities. — Gregory, 1899, classified this genus in the Horneridae. 

 This is correct so far as the exterior aspect and the nature of the 

 tubes is concerned, but the ovicell is quite different and is of the 

 type characteristic of the Ascosoeciidae. 



The genus differs for Laterocavea D'Orbigny, 1853, in the nature of 

 the adventitious pores, which are vacuoles and not mesopores. In 

 the occurrence of large noncellular areas the two genera are very 

 close but the areas are arranged quite differently. 



SipJiodictyum differs from Reteporidea D'Orbigny, 1853, in the 

 nature of the adventitious pores, which are vacuoles placed at the 

 bottom of the sulci, and in its nonreticulate zoarium. 



The physiological function of the vacuoles in recent Hornera is 

 not known, and we are unable, therefore, to understand the impor- 

 tance of the large noncellular spaces on species of Laterocavea and 

 SipJiodictyum. 



Certain branches are petaloporoid, as they have orifices entirely 

 around the zoarium. 



SIPHODICTYUM GRACILE, Lonsdale, 1849 



Plate 14, figs. 14-21 



1899. SipJiodictyum gracile Gregory, Catalogue of the Cretaceous Bryozoa in 

 the British Museum, vol. 1, p. 363, fig. 45, pi. 12, figs. 14, 15. (Bibliog- 

 raphy.) 



Measurements. — Diameter of orifices, 0.08 mm.; diameter of zoo- 

 ecia, 0.16 mm.; distance of orifices along margin, 0.40 mm.; diam- 

 eter of large branches, .070 mm. 



Structure.— The zoarium is cylindrical, dichotomously branched. 

 The peristomes are salient, adjacent, and arranged in transverse 

 rows interrupted in the middle, on the anterior face of the colony. 

 They are separated in the longitudinal direction by 4 to 6 vacuoles. 

 On the posterior face only vacuoles occur, at the base of broad sulcj 

 of little depth; the sulci are rarely longitudinal and are almost always 

 more or less oblique. This arrangement prevents the discovery of 

 the true nature of the pores as observed in thin sections. The dorsal 

 is sometimes much reduced. 



In longitudinal section the zoarium appears surrounded by a very 

 thick lamellar epitheca ; the tubes are short, cylindrical, with peri- 

 stome, recurved at their extremity, with triparietal gemmation, and 

 arranged around a central axis. The tubes oriented toward the 

 frontal are complete and separated in the recurved portions by 

 the epitheca perforated by vacuoles; the tubes oriented toward the 



