44 J- E. DUERDEN. 



is that while the majority pass through their bilateral develop- 

 mental stages and attain practical radiality, some retain the early 

 bilaterality, evidenced by the presence of one or more fossulae, 

 The single stomodaeal groove of the polyp, correlated with an 

 axial fossula in the skeleton, is the only deep-seated character 

 uninfluenced by the equality of the environmental influences, and 

 usually precludes the attainment of perfect radiality among the 

 Rugosa. 



Why should certain rugose corals when mature retain the de- 

 velopmental bilaterality of the Hadi'ophj'lhiin-s,\.a.v& while others 

 continue their growth until their septa attain the radial condition? 

 The former has been shown to be an early ontogenetic stage of 

 all forms, hence rugose corals with alar fossulae and a compound 

 cardinal fossula are at a lower developmental stage than those 

 attaining radiality. How far the differences are chronological 

 seems uncertain, for some of the earlier Rugosa are as nearly 

 radial as their later representatives, and the adult HadropJiyllwn- 

 stage was apparently no more common in earlier than in later 

 Palaeozoic times. According to Zittel ("Text-book of Palaeon- 

 tology," Eng. Ed., p. 74), the distribution of certain genera 

 characterized by a well-marked Hadrophyllum-steige. is as fol- 

 lows : Baryphyllum, Devonian ; Ifadrophyllum, Devonian ; Ani- 

 sophyllum, Ordovician to Devonian ; Menophyllum, Carboniferous 

 Limestone ; Microcyclus, Devonian. Others having a more 

 nearly radial disposition of the septa in the adult are distributed 

 in time as follows : Streptelasma, Ordovician to Carboniferous ; 

 Zaphrentis, Silurian to Carboniferous ; CyatJiaxonia, Carbonif- 

 erous Limestone ; Duncanclla, Silurian ; Palceocyclus, Silurian. 

 Obviously both forms of corallite, bilateral and radial, were liv- 

 ing together during the greater part of the Palaeozoic age. 



When the majority of trifossulate zaphrentoids are compared 

 with the radial forms, the former are seen to be short, flat, or 

 trochoid corallites, the latter long and conical ; also the trifossu- 

 late species are usually smaller and have a fewer number of 

 septa. It would seem that corallites with considerable vertical 

 height have had the opportunity, as it were, to outgrow their de- 

 velopmental bilaterality, and their septa have assumed the radiality 

 of maturity, while this has not been the case with the short flat 



