552' Report of the State Geologist. 



Family Rhabdosemonidae, Yine. 



This family includes forms whose external characters are simi- 

 lar to those of Rhomboporid^, but the cells arise from a filiform 

 axis at the center of the branch. It includes the following 

 genera: Acanthoclem:a, Nemataxis and (?) Tropid jPoba. 



ACANTHOCLEMA, Hall. 



(Pal. N. Y., Yol. YI, p. 7*2. 1887.) 



Type, Acanthoclema alternatum, Hall. 

 (Plate 16, figs. 6-12.) 



Zoarium ramose, branches slender, cells cylindrical, arising 

 from a filiform axis at the center of the branch ; cell apertures 

 oval, arranged in longitudinal parallel rows, separated by ridges 

 or in diagonally intersecting rows ; each aperture enclosed by a 

 polygonal elevation. IS^odes or canical spines usually occur 

 between the cell apertures. When the interapertural space is 

 spinulose the intercellular space is occupied by tubuli formed by 

 the growth of the hollow spines. When the spines are absent 

 the intercellular space is solid. The external characters of this 

 genus and Khombopora are so similar that a knowledge of the 

 internal characters is necessary to distinguish the genera. 



JS'emataxis, Hall. 

 (Pal. N. Y., Yol. Yl, p. 74. 1887.) 

 Type, Nemataxis fihrosa^ Hall. 



(Platel6, figs. 15-18.) 

 Zoariura ramose; cells cylindrical, arising from a central filiform 

 axis, diverging at an angle of forty-five degrees until within a 

 short distance of the surface, when they abruptly turn outward; 

 . the former portion of the cells in contact, the latter slightly sepa- 

 rated; interspace apparently solid; cell apertures oval, arranged in 

 parallel longitudinal rows, separated by a ridge; peristomes very 

 thin, slightly elevated On the surface, at intervals, there are 

 monticules destitute of cell apertures, which extend across the 

 branch and give to it an annulated appearance. Frequently a 

 solid calcareous growth extending over the branches obliterates 

 all traces of the cell apertures. 



