106 Froceedings of the Royal Society of Victoria, 



P. megasio/imt)i, McCoy MS., W. S. Dun, 1898, Proc. R. Soa 

 Vict, Vol. X., New Series, pt. II., p. 83, pi. III., fig. 1. 



Observations. — By comparing the specimens of Pleurodictyumi 

 in the collections at the National Museum, to which Sir Frederick 

 McCoy had attached a MS. name, I am able to verify Mr. Dun's> 

 supposition, that the specimens he figures from Mr. Sweet's- 

 collection are identical with those previously mentioned by 

 McCoy.^ Since, however, this form was neither figured or 

 described prior to Dun's paper the reference remains as above. 



Some of the specimens in the Museum are of large dimensions, 

 a tabula in one [340] measuring 13 mm. in width, and the 

 corallum 55 mm. 



From the very complete examples available, the following 

 diagnosis can be given. 



Description. — Corallum roughly circular, or slightly elongatedi 

 in one direction ; base gently convex, often showing across the- 

 centre a deep impression of the crinoid stem to which it was in 

 the habit of affixing itself. Epithecal layer of base concentrically 

 rugose, marked by thin wavy ridges. On this basal surface the 

 areas of the corallites are bounded by irregular ridges arranged 

 radially from the centre of the corallum. In this latter feature, 

 and also in its more complanate form, P. megastomum differs 

 specifically from P. stylophorum, Eaton. Area of corallum; 

 consisting of from 8 to 14 polygonal corallites united by large- 

 mural pores, numbering about 8 or more in the length of the 

 radial walls in one plane, and rather irregularly disposed. 

 Surfaces of tabulaj pustulose, excepting near the periphery of 

 the outer corallites where the surfaces tend to become radially 

 ridged. Width of corallum in an average specimen, 45 mm.;, 

 height, 10 mm. 



Occurrence. — P. megastomum usually occurs as a cast in the 

 blue or brown Silurian argillaceous limestones ; the rock often, 

 weathering to a bright yellow in parts, especially in the neigh- 

 bourhood of the fossils. One of the specimens alone, from 

 locality B. 23 [341], shows the original form and the epithecal 

 layer, but the crinoid stem to which it was attached has been 

 dissolved away. 



1 Ann. and Mag. Nat. Hist., series 3, vol. xx., 1867, p. 201, footnote. Also, Smyth's. 

 1st Proj^ress Report, Melbourne, 1874, p. 34. 



