CYCLOSTOMATA. 



629 



of two strata of tubular zocecia, the bases of which unite to form a 

 median calcareous striated layer, along the line of which the frond 

 readily splits. The zocecia are disposed in parallel longitudinal 

 lines, those of the central rows being themselves longitudinal, while 

 those of the lateral regions of the colony are directed obliquely 

 upwards and outwards, the arrangement of the cells being thus 

 feather-like (fig. 472). The two flattened surfaces of the polyzoary 

 are wholly covered by the minute, oval cell-apertures. As regards 

 the internal structure, the zocecia have strong, fibrous and imper- 

 forate walls, and the cells are traversed by numerous transverse 

 calcareous partitions or " tabulae," many of which, however, are 

 incomplete, and do not extend the whole way across the tube in 



Fig. 473. — Minute structure of Ptilodictya (Heterodictya) gigantea, Nich., from the Cornifer- 

 ous Limestone (Devonian) of Canada, a, Vertical and longitudinal section, showing the well- 

 developed tabulae ; b, Vertical and transverse section, showing the incomplete condition of many 

 of the tabulae ; c, Tangential section. All the figures are enlarged eighteen times. (Original.) 



which they occur (fig. 473). The cells run obliquely to the 

 surface, and are in contact throughout, no interstitial tubes or 

 cells being developed. 



If the form just described is to be taken as a true Ptilodictya, then 

 the essential characters of the genus Ptilodictya and of the family 

 PtilodictyonidcE are that the polyzoary is leaf-like, and composed of 

 two layers of tubular zocecia, which are attached back to back, a 

 spurious mesothecal membrane being formed by the coalescence of 

 their bases ; the cells are in contact throughout, no interstitial cells 

 being present ; the cell-mouths are simple ; and cross-partitions or 

 " tabulae " are abundantly developed in the tubes. Owing to the 

 uncertainty which at present attaches to the limits of the family 

 PtilodictyonidcE, it is impossible to speak definitely as to the geolog- 

 ical range of the family ; but P. lanceolata, Goldf., is Silurian, and 

 P. (Heterodictya) gigantea is Devonian. 



There are various Palaeozoic Polyzoa that have been commonly 

 placed in the genus Ptilodictya, which agree with the preceding in vari- 

 ous general characters, but differ in important structural features. Thus, 



