76 



G. LINDSTRÖM, HELIOLITID7E. 



This genus is al the earliest found in the lowest Upper Silurian beds continuing 

 prolific in variable forms through the ^vhole of that formation, but not higher, so that 

 not a single specimen is known from the Devonian formation.^ 



In distinguishing and describing the species belonging to this genus or rather in 

 trying to subdivide the various fossil forms pertaining to it, into distinct species great 

 difficulties are to be encountered through their great variability and their similarity in 

 many features. There are at least three groups of species. One contains Pl. petaliformis, 

 foroensis, calyculata and scita with their large spiny septa, the sinuous barbed aureolar 

 radii, the irreguliar coenenchymal tubuli, with spiny walls, and the coenenchyma composed 

 both of convex lamellas and aculfc, bacilli or irregular tubuli enclosing vesicular lengthened 

 traverses. Another group is formed by Plasmopora suprema, rudis and heliolitoides with 

 short, nearly evanescent septa, a narrow aureola, and peculiar funuelshaped and oblique 

 traverses in the irregular tubuli. A third group consists of Plasmopora stella, scala and 

 rosa with large, well developed aureola, simple straight septa and partly with nearly 

 regular Heliolitidean coenenchyma. To these I have joined yet one species, the belonging 

 of which to this genus may be contended, but I can at present not find it in greater 

 aflfinity with any other genus. 



The geological distribution of these species is the following: 



Plasmopora petaliformis Lonsd 



foroensis n 



Upper- 



most 



strata of 



Lower 



Silur. 





p 



o 

 < 



1 



^1 



t» ö 



Ludlow beds. 



a 



b 



c 



d 



e 



f 



g 



h 





* 

 * 



* 

 * 



I 



l 



1 



calyculata n 



var. tuberosa n 



* 

 * 



* 

 * 

 * 

 * 



1 1 



1 



1 



scita M. Edw. & H 



follis M. Edw. & H 



Stella n 







1 

 i 



i 

 1 



scala n 



1 



1 



rosa n 



. 



5f 



* 



* 



* 



* 



sui)rema a .... 



r . 







; 



1 

 1 



? reticulata n 



1 On the prctcndcd Devonian species see at the enrl of this genus. 



