American Palaeozoic Bryozoa. 



163 



series, or in a quincuncial manner ; pseudo-septa arc frequently pres- 

 ent ; the walls in many species are pierced by connecting foramina. 

 True interstitial cells usually absent ; but in the nodose species the 

 summits of the monticules are often occupied by smaller cells than the 

 average. In the robust species the tubes are crossed by diaphragms 

 placed upon the same level in contiguous tubes. 



Ptilodictya maculata, n. sp. (PI. VI., fig. 17, and PI. VII., figs. 4, 4a.) 



Zoarium consisting of a single, unbranched, flattened, two-edged 

 frond, which is more or less curved, and gradually expands from the 

 pointed articulating " head" upwards. The width of the frond above 

 varies in different examples from one half an inch to one and a half 

 inches. The total length may exceed five inches, while the greatest 

 thickness of a robust specimen does not exceed one tenth of an inch. 

 From one to two tenths of an inch above the extremity of the striated 

 and more or less pointed articulating process, the zoarium suddenly 

 expands and forms a kind of shoulder. Cells rhomboidal or hexagonal, 

 with oval or circular apertures, and arranged in intersecting diagonal 

 lines, the regularity of which is interrupted at intervals of about. 1 inch, 

 by' groups of cells of a larger size than the average, which occupy 

 slight elevations of the surface. Between these groups about twelve 

 cells occupy the space of .1 inch. Walls of cells at the surface moder- 

 ately thin. Sections show that the walls are thick and perforated by 

 connecting foramina; and that diaphragms are developed at corre- 

 sponding levels in contiguous tubes. In tangential sections the cells 

 are usually, irregularly petaloid; the pseudo-septa number in each 

 tube from one to five 



The characters of this species are intermediate between those of 

 P. falciformis, Nicholson, and P. pavonia, D'Orbigny. From the 

 former it is distinguished by its mucli more robust fronds, and groups 

 of larger-sized cells. From the latter it is separated by its compara- 

 tively narrow fronds, which never expand so irregularly, nor nearly so 

 much as those of P. pavonia. 



In this connection, it is proper to consider the characters of Ptilo- 

 dictya pavonia, D'Orb., since Dr. Nicholson ( u Monticulipora," p. 196, 

 1881), has questioned my view of the affinities of this species. D'Or- 

 bigny originally referred the species to the genus Ptilodictya, which 

 course I believe to have been unquestionabty correct. In his discussion 

 of the subject, Dr. Nicholson says (loc. cit.): "This beautiful form 

 presents a considerable superficial resemblance to Ptilodictya, and 

 has been referred to this genus. It wants, however, the definitely cir- 



