﻿302 DR. STANLEY SMITH ON AULINA BOT1FOBMIS, ETC. [vol. lxxii. 



1 admit that the prominence of this axis is a characteristic feature 

 of the species. 



The material examined by Schafer included specimens of 

 Orionastrcea ensifer from Bristol, as well as of O. pliillipsi and 

 O. placenta from North Wales and Derbyshire. The Bristol 

 specimen (No. 56740, British Museum), which he figured as 

 ' P. radiata,' could be referred almost equally as well to O. 

 pliillipsi as to O. ensifer; but I consider it to belong to the 

 latter, rather than to the former species. 



O. ensifer is considerably less differentiated from the species of 

 Lithostrotion than is the genotype 0. pliillipsi, and constitutes, 

 as previously suggested, the passage-form between Lithostrotion 

 and Orionastrcea. Characters not described here may be taken to 

 be in agreement with those of the genotype. 



External Characters. (PI. XXIV, fig. 5.) 



The corallum has the same general form and characters as 

 O. pliillipsi, except in regard to the distal surface ; in this it 

 differs very slightly from Lithostrotion hasaltiforme. The indi- 

 vidual calices are clearly defined, and each is divisible into an 

 intrathecal depression and an extrathecal platform. The latter is 

 broad and sloping, and meets that of the contiguous corallites 

 in a sharp ridge. 



In Lithostrotion this ridge is surmounted by a distinct wall of 

 epitheca, and it is in the absence or weak development of this that 

 O. ensifer is distinguished from the massive forms of Litlio- 

 strotion. Calices of the two genera are figured in juxtaposition 

 for the purpose of comparison (PL XXIV, figs. 5 & 6). 



Internal Characters. (PL XXIV, figs. 3 & 4.) 



A thin epitheca may divide the corallites ; but, if present at all, 

 it is usually reduced to a palisading of isolated rods which in 

 transverse section .present a line of dots. The septa never attain a 

 confluent condition. The number of sej)ta present is a fairly 

 constant character : the number, including both series, closely 

 approximates to 36. The major septa extend farther into the intra- 

 thecal region than those of O. pli illipsi generally do, and are much 

 more uniform in their length. The minor septa, on the other hand, 

 are but feebly developed. The columella is persistent and stoutly 

 built, but this axis is not found swollen to the extent sometimes 

 observed in O. pliillipsi. The thecse usually measure about 



2 5 mm. in diameter, but I have examined a specimen in which 

 this measurement attained 4 mm. and in which the extrathecal 

 region was proportionately wide, yet the number of septa present 

 was the same as in the forms of more, usual size. 



