American Palaeozoic Bryozoa. 



151 



two alternating longitudinal series of cell apertures. Non-poriferous 

 side longitudinally striate. Sub-carboniferous and Carboniferous. 



Family Arthronemidce, Ulrich. 



Zoarium dendroid, composed of numerous small, sub-cylindrical seg- 

 ments, carrying cells on one or both sides. 



Arthronema, Ulich. — Segments small, slender, poriferous on one side 

 onlj 7 ; opposite side longitudinally striate. Cells in two to four rows. 

 Trenton and Cincinnati. 



Arthroclema, Billings. — Segments cylindrical, with cell-apertures on 

 all sides. Trenton. 



Sub-order Trepostomata, Ulrich. 

 This sub-order is proposed for the reception of the majority of the 

 Palaeozoic, and many of the more recent Bryozoa. The principal dis- 

 tinguishing features of the sub-order are — (1) that the zoarium is com- 

 posed of slender fasciculate tubes, which do not (as is the case in the 

 Cyclostomata) gradually enlarge as they approach the surface, but re- 

 main throughout nearly of the same diameter; and (2), that at a cer- 

 tain point in the course of the tubes to the surface, they bend outward 

 more or less abruptly, and change in character. Besides the following 

 Palaeozoic families, the Cerioporidce should be referred to the Trepos- 

 tomata. 



Family Ptilodictyonidce, Zittel emend. Ulrich. 



Zoarium jointed, consisting either of a single leaf-like or compressed 

 ramose segment, which articulates with the expanded and attached 

 base ; or of numerous similar segments. The segments are composed 

 of two layers of closely-arranged tubular cells, grown together back to 

 back. No interstitial cells. Diaphragms are often developed. 



(The forms described by me (This Journal, vol. ii., No. 1) under 

 the name of Crateripora, are now known to be the attached bases of the 

 Ptilodictyonidoe. The forms described as C. lineata, and var. expansa, 

 belong to species of Ptilodictya. The bases of Arthropora were called 

 C. erecta. 



Ptilodictya, Lonsdale. — Zoarium below, sub-solid, wedge-shaped or 

 pointed ; above, either an undivided leaf-like expansion, or branching 

 dichotomously. Margin non-poriferous. Cell-apertures quadrate or 

 hexagonal. Trenton to Lower Helderberg. 



Graptodictya, Ulrich. — Zoarium pointed below, branching above. 

 Cell-apertures circular, and separated by interstitial pits or sulci. 

 Cincinnati Group. 



