﻿part 4] PHILLIPS ASTRAL A HENNAH1, AND ORIONASTRCEA. 



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represent the phylogerontic (or senile) stage in the history ^ of 

 that stock. The separation of Lithostrotion ensifer from the 

 other species of that genus and its inclusion here may encounter 

 reasonable objection, since the differences that it exhibits are cer- 

 tainly not great ; but, as it possesses (although in a less-developed 

 condition) those features which distinguish the genotype of 

 Orionastrcea, I believe that the step is justifiable. 



Generic Characters. 



The corallum is composite and massive, and the corallites are 

 either defined by a thin epitheca, or, in the more typical forms, 

 by no epitheca at all ; in this latter case the corallites are united 

 by the dissepimental tissue, and the septa are confluent. Both 

 major and minor septa are well developed, and a columella is 

 present (except in O. placenta). 



The characters of Orionastrcea are essentially those of Litho- 

 strotion, but in a modified form, and their structures are more 

 unstable and variable. Even in the same corallum the difference 

 between the corallites is often very marked, as in PI. XXIV, fig. 2. 



From a large quantity of material examined three distinct types 

 of Orionastrcea can be readily isolated, represented respectively 

 b} r the three species here described ; but a large proportion of 

 the specimens falls between two species rather than within one 

 of them, and might, therefore, be ascribed to the one species 

 equally as well as to the other. 



To 0. ensifer specific rank can with little hesitation be accorded, 

 and it is easily distinguished from O. pliillipsi and O. placenta. 

 Furthermore, it represents almost exclusively the genus in the 

 Bristol area, whereas it is very rare in North Wales, where 

 O. pliillipsi is so abundant and where O. placenta also occurs. 



I have considerable doubt as to whether, in the strict zoological 

 sense, it is correct to regard 0. pliillipsi and O. placenta as 

 separate species : both occur together, and the difference may be 

 nothing more than that between individual colonies of the same 

 species. Since, however, in the absence of sufficient evidence this 

 question cannot be settled, and since it may prove useful to have 

 separate names for the two types, it seems permissible and even 

 desirable to recognize McCoy's species rather than keep them 

 merged under Martin's name ' radiata." 1 



Genotype : O. pliillipsi (McCoy). Type-specimen of O. 

 pliillipsi : No. 213 a, Sedgwick Museum, Cambridge. (Lectotype 

 chosen from two syntypes.) 



Summary of Research. 



The earliest figures of Orionastrcea are those of a highly 

 siliceous specimen from Winster (Derbyshire), published -iby 

 William Martin in 1809, 'Petrificata Derbiensia' pi. xviii, figs. 2 & 3. 



Q. J. Gr. S. No. 288. z ' 



