BKYOzioA. 165 



Ptilodiotya.l 



diagonal arrangement of the zooecia prevails throughout ; so that instead of narrow 

 oblong-quadrate zooscial apertures along the center of the fronds, we have there, the 

 same as on all other parts of the surface, rounded apertures situated in rhomboidal 

 or hexagonal spaces. 



A subdivision of Escharopora is likewise possible, but in this case we make out 

 three instead of two. In the first we have simple narrow zoaria, with the diagonal 

 lines of zooecia extending without interruption, completely across the celluliferous 

 faces. E. recta and Ptilodictya falciformis Nicholson, are good examples. In the 

 second the zoaria are also simple, but wider, occasionally very large (e. g. Ptilodictya 

 pavonia d'Orb.) and at regular intervals their surfaces exhibit clusters of large cells. 

 The latter are commonly elevated into rounded or conical monticules. 



These two subdivisions though obvious enough and in the main indicative 

 of natural relations, are nevertheless not entirely so, since they separate species like 

 P. falciformis Nicholson, and P. maculata, that most certainly are closely allied, and 

 in practice sometimes difficult to distinguish even specifically. Again, we know 

 forms, Escharopora (Ptilodictya) subrecta for instance, in which old examples, or may 

 be they are entitled to the distinction of a variety, have one, two, or even three rows 

 of monticules. On the other hand, I am fully convinced that in the young zoaria of 

 the normally montiferous species, the monticules were, to say the least, a very 

 inconspicuous feature compared to what they are on the fully matured zoaria 

 (See footnote, ante p. 146.) 



The third subdivision includes the branching forms. So far as known, it is a 

 natural grouping, and distinguished from the preceding by the branching of the 

 zoaria, and parallel margins of the branches after the first or basal division. When 

 the branches are wide, clusters of large cells and monticules are developed, but 

 when they are narrow, the monticules are absent, and the large cells distributed 

 along the non-poriferous edges. A subdivision of the branching forms is possible 

 therefore precisely as in the simple species. 



In accordance with the above I offer the following amended definition of Ptilo- 

 dictya and Escharopora, and classification of species. 



Genus PTILODICTYA, Lonsdale, 1839. 



Zoaria bifoliate, simple, umbranched, lanceolate or falciform, terminating below 

 in a solid, striated, pointed base, which originally fitted loosely in the centrally sit- 

 uated cupshaped depression or socket of a small basal expansion. The latter grew 

 fast to foreign bodies, is radially striated, and has small cell openings in the furrows 

 between the striae. In very young examples, and in certain small species in which 



