36 Prof. H. A. Nicholson — New Palaeozoic Polyzoa. 



tial tubuli, similar tubules being present on the striated margins of 

 the branches, and over considerable portions of the footstalk. 



The only example of this species that I have seen is growing 

 upon Heliophyllim Halli, to the exterior of which the footstalk is 

 attached by a widely expanded base. 



Locality and Formation. — Bartlett's Mills, Arkona, Township of 

 Bosanquet, Ontario. In the Hamilton Formation. 



Fenestella Davidsoni, Nicholson. Plate II. Figs. 3-33. 



Frond small, flabelliform, the branches ("interstices") keeled on 

 both sides with very high, thin, and sharp-edged carinas. Three or 

 four branches in the space of one line, dividing dichotomously, 

 usually with great regularity, at intervals of from two to three lines. 

 Both the branches and the keels are more or less wavy or sinuous, 

 sometimes as regularly so as in some Reteporce ; whilst the dissepi- 

 ments are very wide, deeply sunk beneath the level of the celluli- 

 ferous surface of the frond, and presenting the appearance of being 

 formed by anastomosis of the branches. The dissepiments are fully 

 one-third of a line in width, and do not carry cells. The fenestrules 

 are oval, about one-third of a line in length, and slightly less than 

 this in width ; about two of them in the space of one line measured 

 longitudinally, alternately placed in contiguous rows. The cell- 

 mouths are rounded or transversely oval, about three of them oppo- 

 site to each fenestrule. Non-poriferous side of the branches smooth, 

 with the same thin, sharp, and prominent keel as exists on the 

 celluliferous side. 



This species, in its mode of growth and division, as well as in 

 the sharpness of the carina between the rows of cells, strongly 

 resembles Fenestella Milleri, Lonsdale ; but the latter is stated to 

 possess narrow and slender dissepiments, placed two lines apart, 

 with fenestrules five or six times longer than wide, about twelve 

 pores going to a fenestrule. The corresponding characters in our 

 species are so strikingly different, that, in spite of the superficial 

 resemblance, I feel fully justified in separating it from F. Milleri ; 

 and I have great pleasure in dedicating it to my friend Mr. Thomas 

 Davidson, F.E.S. 



F. Davidsoni is distinguished by its regularly dichotomising 

 branches, with prominent sharp-edged keels on both sides ; the 

 undulated character of the branches ; the deeply sunk position and 

 great width of the dissepiments, which carry no cells, and look as 

 if formed by anastomosis of the branches ; and the oval, slightly 

 longer than wide fenestrules. In the general aspect of the celluli- 

 ferous surface and the sinuous course of the branches, the species 

 makes a close approach to some species of the genus Betepora ; but 

 the presence of non-poriferous dissepiments and the existence of a 

 keel separating two rows of cells seem to justify its reference to 

 the genus Fenestella, of which, however, it cannot be regarded as a 

 typical member. The keels are so prominent, that specimens, espe- 

 cially when seen from the non-celluliferous side, often exhibit 

 nothing except the carinas projecting above the matrix. 



Locality and Formation. — Hamilton Group ; Bartlett's Mills, near 



