154 



HARD WICKE 'S S CIENCE - G O SSI P. 



began in the Lower Silurian, having but one repre- 

 sentative ; in the Upper Silurian, six ; in the Devonian, 

 five ; in the Carboniferoiis formation, twenty-two, and 

 in the Permian, one. This was previous to the labours 

 of Professor Nicholson on the Palaeozoic Palaeontology 

 of North America. Since then he has described several 

 species new to the fauna of the Silurian and Devonian 

 era, swelling the list of described species to nearly 

 forty in number — a sufficient evidence that this was 

 the most prevailing genus of the Palaeozoic seas. 

 The signification of the term is, a "little window," 

 and if the reader will bear this interpretation in mind, 

 he will soon get over the difficulty of specific dif- 

 ferences. Let us, then, suppose a common window- 

 frame, minus the glass. The vertical bars will repre- 

 sent the "interstices,'' the horizontal bars the " dis- 

 sepiments," and the open spaces where the glass 

 should be, " fenestrules. " The whole expansion is 

 called a Polyzoary. Pi-ofessor Nicholson, in de- 

 scribing species of Fenestella, calls the vertical bars 

 "branches," and wherever the branches separate, like 

 the prongs of a fork, "bifurcations." 



The Polyzoary of the Fenestella is a calcareous and 

 cellular structure, forming a conical or fan-shape 

 expansion of radiating branches. The external surface 

 of the branches is rounded and covered by a minutely 

 porous layer. The middle portion of the interstices 

 is occupied by a keel (carina) separating two rows of 

 mouths or short tubular cells. The non-celluliferous 

 side is formed internally of a layer of vertical hair- 

 like tubes.* Many of the species of Fenestella 

 encrusting rocky masses of the Silurian or the 

 Devonian seas are known only by the non-poriferous 

 side, as also many species of the carboniferous lime- 

 stone. Whenever the celluliferous side is shown, the 

 cell-mouths are either round or slightly oval. In 

 section, the cells may be said to have an outer and 

 an inner chamber, but are really the continuous cell 

 in an oblique line. In the longitudinal section, pre- 

 pared for microscopic examination, the keel is a thin 

 wall with a sinuous outline, which occupies the whole 

 length of the branch, separating the two rows of 

 pores which nin along the border-lines of the fenes- 

 trules. Evei-y cell is independent of the other, and 

 the living animal, which at one time occupied the cell, 

 was as distinct in its habitation as are the cyclosto- 

 matous Polyzoa of the present time. The dissepi- 

 ments are, apparently, continuations of the border- 

 line of the fenestndes, forming a part of the cccnecium 

 only, and are entirely destitute of pores. The exact 

 fonn of the cell is not peculiar to Fenestella, but there 

 is a generic difference in the manner of development 

 of the cell along the whole length of the branches 

 which separate the species of this genera from Rete- 

 pora, although there may be a resemblance in the 

 facial configuration. My experience leads me to 

 believe that true Retepora are far from common in 



*Sce M-^Coy's "Carb. Fossils." 



the carboniferous limestone ; and to study the 

 affinities of the genera I have had to have recourse 

 to sections of species from the Coralline crag of Suffolk. 

 In Retepora the cells are in oblique lines, and the 

 fenestrules, on account of their irregularity, are pro- 

 bably accidental in part, whilst in Fenestella the shape 

 of the fene.strule has often been taken as the ground- 

 work of specific distinction. The development of 

 Fenestella, as also the development of Polyzoa 

 generally, was from the base outwards by means 

 of bifurcations of the branches. In one particular 

 species {F. forinosa), at every bifurcation of the 

 branch, a cell, larger than the ordinary cells, M'Coy 

 says,* is formed in each angle. t In the specimen that 

 I am now describing, from Hurst, in Yorkshire, this 

 angular cell is formed within the immediate angle 

 of bifurcation, having a small tubercle where the 

 branches join. A narrow keeled dissepiment unites 

 the interstices, and above this two cells, the walls of 

 which come close together without any intercellular 

 space, form the real base of the newly-rdeveloped 

 fenestrule ; above this, four circular cells are formed 

 on each side of the interstices of the fenestrule, about 

 half the cell's diameter apart. This regularity is, in 

 many cases, uneriing ; but the animals were not 

 always regular in their continuous operation, for at 

 one bifurcation in the same specimen no fenestrule is 

 formed, and the slightly alternate cells are developed 

 in one plane, the interstices of the non-poriferous 

 side being likewise double. I notice another pecu- 

 liarity in this species, but in another specimen. The 

 first-formed fenestrule above the root is destitute of 

 cells, and even the fenestrule above this is destitute 

 of cells on the interstices fully a third of the distance 

 upwards, and the uninterrupted branches are covered 

 with closely-set strips, bending in a spiral round the 

 branch. 



Another species of Fenestella found at Hurst, in 

 Yorkshire, will answer to the description given of 

 F. bicellnlata, a new species of Fenestella found in 

 fragments only in the Calderwood limestones of 

 Western Scotland. The specimen measures a quarter 

 of an inch by three-sixteenths. The celluliferous, as 

 well as the non-celluliferous aspect, is distinctly 

 visible. In the limits of this space I can count 41 

 fenestrules, besides several halves. The enlargement 

 of this species is by bifurcations, and the fenestrules 

 on the non-poriferous side are very much like the 

 Devonian species figured in Science-Gossip for May 

 (fig. loi), but considerably smaller, and the branches 

 rounded. The Polyzoarium is flattened and ex- 

 panding. The interstices are straight, carinated 

 (keeled), occasionally bifurcating, and the whole 

 carina is ornamented by prominent pores. There is 

 no regularity in the arrangement of these pores ; 

 occasionally one pore is placed at the end of each 



* " Carboniferous Fossils." 



t In several specimens that I have in my cabinet the|cell of 

 the angle is the same size as the other cells. 



