108 



tical septum, traversing the short diameter of the frond, and prolonged on either side into a 

 keel. Leaving this point, however, out of the question, as uncertain, there remain the fol- 

 lowing differences : 1. In Ptilodictya the cells are not furnished with prominent mouths, 

 but with depressed apertures, and these are arranged in rows, which are separated by distinct 

 thread-like elevated lines or striae. On the other hand, the cells of Tceniapora have pro- 

 minent pustuliform openings, and they are usually not separated by any lines or ridges of 

 any kind. Tn some examples from the Hamilton group of the State of New York, the 

 first row of cells on either side of the keel has a limiting thread-like line, but I cannot 

 detect any such structure in our Canadian specimens ; and even the former have the remain- 

 ing rows of cells not marked off in this way. 2. There is no mesial keel or elevated ridge in 

 Ptilodictya, separating the coencecium into two symmetrical lateral halves. On the other 

 hand, the most conspicuous feature in Tceniopora is a strong elevated longitudinal carina on 

 either side of which the cells are arranged in alternating lines. 3. Whilst some examples of 

 Tfeniopora show a distinct non celluliferous marginal zone on either side, as exists in Ptilodictya, 

 others do not appear to possess any such structure ; and in any case this zone appears, when 

 present, to be smooth and not striated. Upon the whole, therefore, whilst recognizing the 

 general affinities of Tceniopora to Ptilodictya, I think the former may safely be regarded as a 

 distinct genus. 



The following two species of Tceniopom have come under my notice as occurring in the 

 Hamilton group ; but as all my specimens are fragmentary, there are several important points 

 connected with their structure, which I have found myself unable to elucidate. I have also 

 specimens from the same formation in the State of New York, some of which appear to be 

 identical with one of our Canadian species, whilst others present certain differences which may 

 perhaps be of specific value. 



132. TCENIOPORA EXIGUA (Nicholson). 



Ta'niopora exigua (Nicholson), Geological Magazine, March. 1874. 



Polyzoary forming flattened linear expansions, which branch dichotomously at mgles of 

 about 60 and at intervals of from one and a half to three lines. The width of the frond is 

 about a line or a little over, and its thickness in the centre is about one-third of a line, from 

 which point it rapidly diminishes in thickness, until the sharp edged lateral margins are 

 reached. Both sides of the ccencecium carry a well marked longitudinal ridge or keel, which 

 occupies a mesial position, and on either side of which are three or four longitudinal rows of 

 cells. The cells are alternately disposed in contiguous rows, so as to form a series of short 

 transverse obliquely ascending lines, consisting each of three or four cells. The cells are im- 

 mersed in the substance of the ccencecium, and their mouths are circular and prominently 

 elevated above the general surface. About six cells occupy the space of one line measured 

 longitudinally, and the intervals between 'them are equal to or slightly greater than the dia- 

 meter of the cell-mouths themselves.- The cells extend on either side of the* midrib, quite to 

 the edge of the coencecium, and there appears to be no marginal non-celluliferous zone. 



From the following spe- 

 cies, Tfeniopora exigua is 

 distinguished by its much 

 smaller width, its more fre- 

 quent division, its much 

 less strongly elevated me- 

 sial keel, and the apparent 

 absence of any distinct non- 

 celluliferous marginal area. 

 One specimen exhibits at 

 the base a small conical 

 Fi 47 expansion from which the 



Toeniopora ?a%wa(Nich). a. Portion of a frond, natural size ; 6. The same enlarged, blanches proceed dlStally, 

 the portion drawn in outline being conjectural ; c. Portion of the same still further en- and which terminates prOX- 

 larged. From the Hamilton group. ,, . .j 



imally in a singular, and 



clearly natural, circular perforation of about one-fourth of a line in diameter. There are 

 also indications in this specimen (fig. 476.) that the branches of the frond were bilaterally or 



c 



