STRUCTURE OF STROM ATOPOROIDEA 59 



genus, are broken through by pillars (Labechia), show reduced pillars 

 on the convex side (Rosenella), or are free from pillars (Beatricea) . 

 Here is to be noted also, as an important negative characteristic, 

 the absence of astrorhizae. 



When, therefore, the families Labechidae and Idiostromidae 

 are thus removed, the Stromatoporoidea no longer stand as a 

 collective group of elements of unequal value, but the remaining 

 "true" Stromatoporoidea represent a clearly limited order, for 

 which the following definition may be given: The skeleton is 

 built of seamlessly united, massive or porous calcareous fibers, 

 which form a more or less regular network. In this can be recog- 

 nized a more or less plainly layered arrangement of the tangential 

 elements. Astrorhizae are always present. 



From the examination of a greater mass of material it appears 

 that in the skeletal structure of the Stromatoporoidea all transi- 

 tional stages from regularly netted (reticulate) to markedly 

 vermiform (vermiculate) are present. The division into a 

 " hydractinoid " and a " milleporoid " group is incorrect, since it 

 is founded upon a wrong supposition, for in none of the forms 

 which were put into both groups are "zooidal tubes" analogous to 

 those of Millepora present. In fact, G. Steinmann in 1903 (Mille- 

 poridium, etc.) and W. Parks in 1909 (Silurian Stromatoporoidea, 

 etc.), the former strongly, the latter at first doubtfully, questioned 

 the so-called "zooidal tubes." 



A still sharper division into two groups lies in the fact that the 

 relatively fine fibers of the family Actinostromidae are massive, 

 while the somewhat thick fibers of the family Stromatoporidae 

 show pores and little canals. On this ground a division of the 

 Stromatoporoidae must rest. 



This division principle: "Fibers massive" — -"Fibers not mas- 

 sive" permits of an unquestionable division. Moreover, it is in 

 harmony with the fact that in the massive-fibered group belong 

 only entirely rectilinearly and altogether regularly built forms, 

 while to the hollow-fibered group may be referred only those forms 

 which are in some way irregularly vermiculate in their skeletal struc- 

 tures. The further division into genera rests in both groups on 

 the degree of regularity of the skeletal mesh. It is, however, no 



