and species of Silurian Radiata. 279 



/ffytertrsif) Petraia aquisulcata (M'Coy). 



Sp. Char. Conical, slightly curved, oblique, with a few broad, ob- 

 tuse, undefined concentric swellings of growth ; regularly in- 

 creasing from the apex to a diameter of 1| inch at 2^ inches 

 from the base ; external wall very thin, it and the cast regularly 

 marked with equal obtuse ribs, about eighty-five in the adult 

 diameter of 1^ inch (six in 3 lines), eighty at 1 inch (eight in 

 3 lines), seventy-five at 9 lines and forty- six at 4 or 5 lines 

 (ten in 3 lines), separated by thin, equal, equidistant slits 

 representing lamellae, each alternate one of which is merely 

 marginal, the others occasionally and regularly uniting before 

 reaching the centre, round which they are twisted to form a 

 spirally conical central area ; no connecting vesicular plates. 



The regularity and equality of size of the lamellar ribs both 

 on the exterior and on the cast, as well as their large number, 

 give a peculiar aspect to this species. In parts of some speci- 

 mens the lamellar sulci are bent in a zigzag manner, but it is an 

 unusual appearance, for which I cannot account. By carefully 

 removing part of the outer wall, each of the lamellse is seen to be 

 split near the exterior (not visible on casts), and each to be per- 

 forated by tubuli, as in Mr. Lonsdale's supposed genus Try- 

 plasma; these punctures leave no trace on the equal, obtuse, 

 smooth ridges of the cast. I have usually seen this coral ticketed 

 in collections as Cyathophyllum turbinatum, to which it bears some 

 superficial resemblance, but has no real specific or even generic 

 relation. 



Very abundant in the Coniston limestone of Coniston ; in the 

 calcareous flags of Applethwaite Common ; fine Caradoc sand- 

 stone of Mulock quarry, Dalquorhan; flags of High Haume; 

 slates of Llansaintfraid. 



Petraia subduplicata (M'Coy).^^^ Unj^o^ Arfi 



Sp. Char. Regularly conic, slightly oblique and curved at the 

 tip ; outer wall thick, marked with one or two small interrup- 

 tions of growth, and with fine, longitudinal, distant impressed 

 equal striae (seven in 2 lines, at a diameter of 5 or 6 lines) ; 

 average length 9 lines, of which the 3 lines nearest the apex 

 are filled with solid calcareous deposit ; width 7 lines. Casts 

 regularly conic, subtruncate at small end; length and width 

 about equal, regularly sulcated by twenty-seven to twenty- 

 eight strong, straight, primary lamellar sulci extending to the 

 centre, where they are irregularly complicated into a large cel- 

 lular mass ; between the large sUts are an equal number of 

 .finer ones, extending about three-fourths the length of the 



