333 



CHARACTERS AND DIVISIONS OF ALCVONARIA. 



rent coalescence of a number of the small tubes does not really 

 militate against the view that the latter are of the nature of 

 "siphonopores." 



As regards their distribution in time, all the members of the 

 Heliolitidce are Palaeozoic, and all are confined to the Ordovician, 

 Silurian, and Devonian periods. 



The type-genus of the Heliolitidce is Heliolites itself, in which the 

 corallum is massive or branched, and the siphonopores are polygonal, 

 and usually regular in form, and possess complete walls (figs. 216, 217). 

 The species of Heliolites range from the Ordovician to the Devonian, 

 and are very abundant in the Silurian rocks proper. 



In the genus Plasmo-bora (fig. 218) the corallum is like that of 

 Heliolites in general structure and appearance, but the walls of the 



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Fig. 218.— A, Transverse section of a specimen of Plas7)iopora petaliformis, Lonsd., from the 

 Wenlock Limestone of Gotland, enlarged five times ; B, Transverse section of Plasmopora scita, 

 E. and H., from the Wenlock Limestone of Gotland, enlarged five times; c, Vertical section of 

 the same specimen, similarly enlarged. (Original.) 



siphonopores are incomplete or obsolete, and their tabulae are thus 

 enabled more or less largely to coalesce, and give rise to a vesicular 

 tissue of lenticular cells (fig. 218). There are generally from two to five 

 rows of siphonopores between adjoining autopores, and the septa may be 

 either lamellar or spinulose. The species of Plasmopora are principally 

 found in the Ordovician and Silurian rocks, but Devonian forms are also 

 known. The Silurian deposits also contain various corals closely related 

 to Plasmopora, for which the names of Propora and Pinacopora have 

 been proposed, but these titles cannot be considered as of more than 

 sub-generic value. In both these groups the siphonopores are much 

 reduced in number, and the autopores are correspondingly close-set ; the 

 calices of the autopores being elevated above the general surface in 

 Propora, but flush with the surface in Pinacopora, and the corallum in 

 the latter being typically discoid in form. 



