98 



ness of about half a line ; dividing dichotomously at angles of 50 or 60, generally at inter- 

 vals of from four to six lines. No developed non-poriferous margins, the whole exterior surface 

 seeming to be occupied by the cells. Cells tubular, curved, opening by oval mouths, arranged 

 in rows which are separated by delicate thread-like lines. About ten rows of cells in the 

 space of one line, the cells of each row alternating with the next, so that the cell-mouths have 

 a quincuncial arrangement. About five cell-mouths in the space of one line, measured longi- 

 tudinally, the interspaces between them being generally considerably longer than the long 

 diameter of the cell-mouths themselves. The partition or laminar axis between the cells on 

 the two sides of the polyzoary is marked with delicate longitudinal striae, which correspond 

 with the rows of cells, together with obscure concentric wrinkles, the convexity of which is 

 directed towards the apex of the frond. 



This species presents a close resemblance to 

 Ptilodictya Gilberti, (Meek), from the Corniferous 

 Limestone of Ohio (Geol. Survey of Ohio, Palae- 

 ontology, Vol. I., Plate XVIII., Fig. 1,) with 

 which I was at first disposed to identify it. I 

 have, however, been compelled to separate it as 

 a distinct species upon the following grounds : 



n A -rwi pyji.j 1 . P. Gilberti is stated to have eight or ten rows 



of cells in the space of two and a half lines ; 

 whereas P. Meeki has never less than ten in the 



Ptilodictya Meeki. (Nicholson) ; a, Portion of the poly- .. ,. ,, . 



zoary magnified to show the branching, and the lon&itu- space 01 One line, Which WOUld give twenty-tive 

 dinally striated 'axis ; ^ A fragment enlarged, showing the , , i if i- n mi P p n | ra | 



form and arrangement of the cells ; c, Transverse section, rows ln two ancl a nal] lmes - z - ^ ne 



magnified. From the Corniferous Limestone. laminar axis of P. Gilberti is said to be covered 



with regular, arched transverse striae; whereas that of P. Meeki exhibits well defined longitudinal 

 striae, with either no transverse striae, or with but obscure transverse wrinkles. 3. P. Gilberti, 

 in common with the typical species of the genus, possesses well-marked striated and non-pori- 

 ferous margins ; but I have been unable to detect any traces of these in the numerous speci- 

 mens of P. Meeki, which have passed under my notice. 4. P. Gilberti seems to be upon the 

 whole a larger and more robust species, sometimes attaining a width of three lines, whereas the 

 average width of P. Meeki is only about one line or a little over. I have, therefore, thought 

 it safest to separate P. Meeki under a distinct specific title, and I have named it in honour of 

 Mr. F. B. Meek, one of the most distinguished palaeontologists of America. 



In some respects P. Meeki presents considerable resemblance to P.fenestrata (Hall), from 

 the Cbazy Limestone ; but in the absence of any sufficiently detailed description of the latter 

 species, and especially of accurate measurements, it is impossible to say how close this resem- 

 blance may really be. 



So far as at present known, P. Gilberti (Meek), and the present species are the only forms 

 of the genus Ptilodictya, which have hitherto been detected in the Devonian Formation. 



Locality and Formation. Common in the Corniferous Limestone of Port Colborne. Rare 

 in the Hamilton formation of Bartlett's Mills, near Arkona, Township of Bosanquet. 



Genus POLYPORA (McCoy). 



"'Corallum forming a delicate, reticulated, calcareous expansion, usually fan-shaped ; 

 interstices " (branches) " round, having on one side from, three to five rows of cell-openings, 

 the margins usually not projecting ; interstices connected by thin, transverse, non -poriferous 

 dissepiments ; reverse rounded, striated, or granulated " (McCoy, Pal. Foss. p. 115). 



According to the above definition, Polypora, is distinguished from Retepom by the posses- 

 sion of distinct dissepiments, which do not bear pores ; whereas in the latter genus the 

 branches inosculate directly, without the development of distinct dissepiments, and the points 

 of junction of the branches carry cells. There are, however, certain forms, such as P. (Gorgonia) 

 ripisteria (Goldf.) and P. gradlis (Prout.), in which the frond resembles Fenestella and the 

 typical forms of Polypora, in having the branches united by distinct dissepiments, but these 

 dissepiments carry pores, whilst there are three or more rows of cells on the branches. It is, 

 therefore, necessary either to constitute a new genus for the reception of these forms, or to 

 admit that the dissepiments of Polypora may carry pores. If the latter course be adopted, 

 Polypora will agree with Fenestella in the general structure of the polyzoary, but will be dis- 

 tinguished by the greater number of the rows of pores to each branch, and by the absence of 



