630 



M0LLUSC01DEA. 



there are numerous Ordovician and Silurian types, of which we may 

 take Plilodictya {Stictopora ?) falciformis as an example, in which the 

 polyzoary is in the form of an unbranched, or slightly branched, flattened 

 frond (fig. 474, a\ composed of two layers of cells placed back to back, a 

 more or less recognisable calcareous membrane (the "mesotheca" of 

 Hall) being produced by the coalescence of their bases. In the par- 

 ticular type here selected as an example of this group the polyzoary is 

 usually unbranched, sickle-shaped, and thin-edged, the margins being 

 longitudinally striated, and occasionally perforated by the apertures of 

 minute and imperfect cells. The zocecia (figs. 474, c, and 475, a) are tubu- 

 lar, and in close contact throughout, no interstitial cells being developed ; 



Fig. 474. — Ptilodictya {Stictopora ?) 

 falciformis. a, Small specimen of the 

 natural size ; b, Cross-section, showing 

 the shape of the frond ; c, Portion of the 

 surface, enlarged. Trenton Limestone 

 and Cincinnati Group, America. (Or- 

 iginal.) 



Fig. 475. — Structure of Ptilodictya {Sticto- 

 pora?) falciformis. a, Tangential section, en- 

 larged, showing the lozenge - shaped tubes ; B, 

 Transverse section, enlarged, showing the arrange- 

 ment of the two layers of cells. (Original.) 



and the apertures are oval or lozenge-shaped. The tubes (fig. 475, b) 

 are rectangular to the flat surfaces of the polyzoary, and they do not 

 appear to be traversed by " tabulae." An essentially similar structure is 

 exhibited by the Silurian Polyzoan described by Hall under the name of 

 Clathropora frondosa, which constitutes the type of the genus Clathro- 

 pora. In this case, however, the polyzoary is composite, and is formed 

 by the inosculation of a series of thin leaf-like bilaminar fronds, which 

 anastomose in such a way as to leave regularly placed oval perfora- 

 tions or fenestrules ; the minute structure of the tubes resembling that 

 observed in Ptilodictya falcifomnis. 



A number of Ordovician, Silurian, and Devonian Po/yzoa, more 

 or less closely allied in general characters to the types treated of 

 above, have been placed in the family of the Stictoporidce. It does 

 not, however, appear possible at present to give any precise defini- 

 tion of the family, since Hall and Ulrich, who have more especially 

 studied the forms in question, are not agreed as to the limits of the 

 genus Stictopo?-a, the type of the whole family. The forms which 

 have been generally included under the name of Stictopora are 

 compressed or leaf-like Po/yzoa, consisting of two layers of cells 



