136 W. A. Parks — Parasite from the Devonian Pocks. 



of structure. The finer one is more abundant and better pre- 

 served ; it is therefore taken as a type of the genus and is 

 described below in some detail. 



The cast of the Platyostoma is seen to be covered with 

 round white spots, separated by narrow brownish lines. Three 

 of these spots with the intervals between them occupy the space 

 of one millimeter. They are regularly arranged in lines in- 

 clined at a slight angle to the sutures of the host. A line may 

 also be drawn through the centers of the spots inclined at an 

 angle of 60 degrees to the first. On close examination these 

 white spots are found to consist of three elements, separated by 

 minute clear lines. These lines are seen to be uniform in 

 arrangement, being approximately parallel in all the spots; 

 see figure 1. Sections a little higher in the organism show 

 that the pillars decrease in size with a corresponding increase 

 in the intervening matter ; they also approach a roughly trian- 

 gular appearance. Still higher, the contiguous angles of adja- 

 cent pillars reach out and unite with each other so as to pro- 

 duce an hexagonal ring with the clear lines of the original 

 pillars forming a continuous line around the middle of the 

 ring. The open center of the ring is occupied by the matrix. 

 It is apparent that each alternate angle of the hexagon repre- 

 sents the position of a pillar, while the other angles are situated 

 at the points of coalescence of adjacent pillars. Higher sec- 

 tions show that this reticulation is continued to the surface of 

 the specimen where the white matter forming the borders of 

 the rings unites and covers the central clear portion. The 

 structure of the reticular network is shown in fig. 2. Tan- 

 gential sections very near the surface of the reticulation show 

 minute points projecting into the openings of the mesh sug- 

 gesting the septa of a favositoid coral or certain of the hydro- 

 corallines. A second variety is seen to be represented by an 

 almost similar structure except for the fact that the pillars on 

 ascending become markedly triangular in outline and extremely 

 regular in arrangement. The cross section of this variety at 

 the level where the pillars approach each other is seen in fig- 

 ure 3. The structure of the network is the same as in the first 

 variety. It is quite possible that these two are identical, and 

 that better preservation in the one case has resulted in the re- 

 tention of the sharp triangular outline of the pillars. It is 

 thought better to include them both in one species. 



There is, however, another species, characterized by a much 

 larger mesh in the network and by the fact that the meshes 

 are rhomboidal in outline instead of hexagonal. The ascend- 

 ing pillars show the same three fine clear lines separating the 

 element, but, at a certain level, there is a tendency for one of 

 these lines to become shortened, so that, on the coalescence of 



