164 THE PALEONTOLOGY OF MINNESOTA. 



[Ptllodlctya. 



The divisions are perfectly natural, and each based upon readily detected pecu- 

 liarities. In the first, including P. lanceolata Groldf., sp., and therefore Ptilodidya in 

 the strict sense about to be proposed, we have a character that is wanting in the 

 second: Namely, a variable number of regular longitudinal rows of zooecia running 

 through the center of the fronds from the pointed articulating base upward. In the 

 earliest species having this peculiarity, these longitudinal central rows do not always 

 extend through the frond to its upper extremity, but they are sometimes found to 

 pass into the diagonal arrangement prevailing over the lateral portions of 4;he surface. 

 In P. magnifica Miller and Dyer, for instance, the longitudinal rows obtain only in the 

 middle of the lower half of the full grown zoaria, the diagonal arrangement being 

 present on all other parts of the surface. d^ 



These central zooecia are oblong-quadrate in shape, narrower than the lateral 

 ones, and always the first to be developed. In the youngest examples of all the 

 species they alone occur, and it is only in later stages that the differently arranged 

 and wider lateral zooecia are developed. It is possible that this condition, which, as 

 said, is an immature or youthful one in most of the species, may have persisted in 

 some, and that in these no lateral zooecia were produced. P. gladiola Billings, and 

 P. ftagellum Nicholson, may be said to support this view, only longitudinal zooecia 

 being as yet known of them. Still, as the evidence is merely negative, and in the 

 light of facts brought out in a study of complete suites of P. variabilis of the Hudson 

 River group, I am obliged to regard the matter as doubtful. 



Used in this restricted sense Ptilodidya admits of subdivision into two groups, 

 both obvious enough, but, as they now appear to me, not quite natural. In the 

 first, with P. lanceolata as the type, we have either nothing but longitudinal rows of 

 zooecia, or these are fianked on each side by spaces of greater or less width 

 over which the apertures are arranged in an oblique manner, giving the fronds the 

 fancied resemblance to a feather that suggested the generic name. The lateral rows 

 .proceed to the edges of the zoarium without interruption from either groups of 

 large, cells, monticules, or maculae. 



In the second subdivision, and of this P. magnifica M. & D. may be considered 

 as typical, the zooecial apertures on the lateral extensions of the zoarium are 

 arranged in diagonally intersecting series, with clusters of large cells, monticules, 

 or maculae, at regular intervals. The pinnate arrangement of parts prevailing in 

 the lanceolate subdivision is therefore scarcely recognizable in this, but the presence 

 of monticules is an even more striking peculiarity. 

 In the second division, for which I propose to adopt Hall's name Escharopora* thq 



* I have some slight doubts respecting the specific characters of E. reota. Hall, the original type of the genus, but norie 

 wljatever so far as its generic characters are ooncerijed, 



