W. A. Parks — Parasite from the Devonian fiocJes. 139 



and in a more and more irregular manner as it increases in age. 

 Further, the meshes of the network in Hydractinia are practi- 

 cally without any tendency to form tubes, i.e. have no vertical 

 extent. No evidence of polymorphism is to be observed in the 

 species under review. Another point of similarity is seen in 

 the fact that the base of a Hydractinia colony shows a more 

 resistant brownish layer where it is attached to the host ; the 

 brown matter lying between the bases of the pillars in our spe- 

 cies may well be analogous to this structure in Hydractinia.'* 

 The presence of the minute septa-like projections near the sur- 

 face of the network suggests that the openings of the meshes 

 were occupied by individual hydroids with very short vertical 

 extent. Such an interpretation would place the creature near 

 to Professor Duncan's Stoliczkaria. This relationship can, of 

 course, be maintained only on the supposition that the coenen- 

 chyma of Stoliczkaria is entirely unrepresented in our species.f 



There is little doubt, therefore, that the new genus may be 

 safely placed under the Hydroida with a considerable resem- 

 blance to Hydractinia and a suggestion of the structure of 

 Stoliczkaria, If this relationship is accepted, the study of the 

 present species throws considerable light on the manner in 

 which the horizontal elements arise from the ascending pillars 

 in all this class of creatures. Further, we have but to assume 

 the closing in of the meshes and the repetition of the process 

 of growth to arrive at the origin of the skeleton of that large 

 and diverse group, the Stromatoporoidea. 



The preferential parasitism exhibited by this organism at so 

 low a stage as the base of the Devonian is a matter of interest. 

 Further, if we accept its hydroid affinities, and omit the 

 graptolites and stromatoporoids, we have in this creature the 

 earliest of the Hydroida, excepting, of course, the very doubt- 

 ful Corynoides and Palseocoryne.;}: 



Tristylotus gen. nov. 



Encrusting parasite on species of Platyostoma. Skeleton 

 probably chitinous or partially calcified, consisting of a continuous 

 polygonal reticulation, supported on short pillars. These pillars 

 are composed of three elements, are sub-circular at the point of 

 attachment and triangular above. The reticulation arises by the 

 drawing out of the angles of the triangular pillars and the 

 coalescence of the points thus produced. 



Tristylotus — vtvXwtos, supported on pillars. 



* Nicholson & Lydekker — Manual of Palaeontology, Vol. I, p. 198. 

 f Ibid., p. 228. 



J Kay Society for 1870. Monograph of the Tubularian Hydroids, George 

 James Allnian, M.D., p. 170 et seq. 



