1^0 W. A. Parks — Parasite from the Devonian Rocks. 



Tristylotus hexagonns sp. nov. 

 The meshes of the network are hexagonal and small. 



Tristylotus rhomboideus. 

 The meshes of the network are rhomboidal and larger. 



The writer is indebted to Professor Eamsay Wright of the 

 University of Toronto for suggestions regarding the affinity 

 of Tristylotus and for the coining of the generic name. 



DESCRIPTION OF FIGURES (p. 137). 



Tristylotus hexagonus 



Fig. 1. — Base of the parasite as it appears on the external cast of Platyo- 

 stoma. Enlarged 30 times. 



Fig. 2. — Diagram to show the origin of the hexagonal network from the 

 triangular pillars. The orginal position and the outline of the pillars are 

 shown in dotted lines. The fine lines show the clear central portion of the 

 mesh. Enlarged about 75 times. 



Fig. 3. — Cross section of the pillars in one variety at a level just below 

 where they begin to unite. Enlarged 75 times. 



Fig. 6. — (a) Vertical section through a pillar in a direction across the web 

 of the network, (b) Vertical section through two pillars in a direction 

 following the web of the network. Dotted part is the clear central portion. 

 Striated part represents the white matter. Enlarged 30 times. 



Tristylotus rhomboideus 



Fig. 4. — Tangential section through the reticulated portion. The positions 

 of the centers of the pillars are indicated by small crosses. The dotted 

 portion represents the clear tissue and the striated portion the white matter. 

 The openings in the meshes are shown round in the diagram ; this is not 

 strictly correct, as they conform more or less to the shape of the mesh. 

 Enlarged about 60 times. 



Fig. 5. — Diagram to illustrate the origin of the rhomboidal meshes. The 

 pillars are indicated by heavy lines. The dotted lines represent the clear 

 lines dividing the pillars into three parts. One of these lines is seen to be 

 much reduced in length. It is this shortening that results in the formation 

 of rhomboidal instead of hexagonal meshes. The outlines of the resulting 

 rhomboidal meshes are shown in fine lines. 



