PROBLEM OF THE MONTICULIPOROIDEA l6l 



except reference to some unknown particular cause if considered 

 as accessory opercular processes. I shall add another hypoth- 

 esis, viz., that they are reduced structures homologous with 

 regular tabulae similarly situated in other species, e. g., Pachydictya 

 foliata Ulr. Their position may be accounted for as related to 

 the sharp turning of the zooid and its cell from axial to 

 peripheral direction, and thus may be taken as evidence that 

 the zooid evacuated the cell as it built. (See text Fig. 2, A, 

 p. 169.) 



Some smaller closely related species {Arthropora) are many- 

 jointed, partly simulating, therefore, the following one. Very 

 probably some cylindrical forms like this one should be consid- 

 ered as to origin as narrow bifoliate, and hence having become 

 rounded. 



Arthroclema armatum Ulr. consists of numerous small cellu- 

 liferous jointed segments which formed a branched zoarium. 

 Numerous primary segments form the main stem, from which 

 lateral branches arise, these being composed of many secondary 

 segments and bearing likewise lateral branches of tertiary 

 segments. These hundreds of segments are each about 3 mm 

 long, subcylindrical with generally six or five or four rows 

 of cell apertures, six to ten apertures in a row. The primary 

 segments are i mm or less in diameter, the others 0.5 and o.3 mm 

 or less. 



Each primary segment has a small axial region in which the 

 cells are prostrate, thin-walled, and angular, and a peripheral 

 region which is thickened according to age. The small, rounded 

 cell apertures are separated by solid, longitudinally furrowed 

 interspace and a large prominence behind each aperture. Some 

 of the apertures are closed by constriction near the articulation 

 areas. The ends or articulation surfaces are solid-faced, respect- 

 ively concave and convex, and the lateral articular socket, when 

 present on a segment, is impressed in the peripheral solid wall. 

 Thin section shows the walls thoroughly amalgamated, the 

 peripheral region merely having striation parallel to the surface. 



