part 3] AND STRUCTURE OF THE EORAMINIFERAL SHELL. 



207 



particles must be directed radially. In the outer whorls this 

 direction is often reversed, and the axes are tangential. The thin 

 walls of the outer whorls possess a structure which is best revealed 

 with the aid of the selenite-plate : it ajDpears as a fine transverse 

 striation due to the alignment of granules having the same optical 

 orientation ; the optic sign may be longitudinal or transverse, more 

 commonly transverse. With non-polarized light the striation is 

 but faintly suggested, though occasionally a single stria is sufficiently 

 obvious. 



The marginal wall consists of a middle layer which forms the 

 greater part, nearly the whole, having, in the outer whorls at least, 

 the optic axes tangential, and two superficial layers, an inner and 

 an outer, in which the optic axes are radial. The lines of growth 

 a]'e visible between crossed nicols, they curve obliquely forwards, as 

 though the opening of the mouth were rostrate. Sometimes a line 

 of growth is emphasized by the extension along it of the inner 

 layer, which may traverse the whole thickness of the wall and 

 become confluent with the outer layer. 



Spirlllina stands in remarkable contrast with the preceding 

 forms ; its shell is indeed fertile in surprises. It is and has long 

 been regarded as a member of the Perforata, a position which 

 its strangely vitreous character would suggest. As such I have 

 always regarded it, and was therefore quite unprepared to find that 

 it might be otherwise than it seems. 



The fine series of Spirillina , numerous in specimens and species, 

 on which my observations are based, I owe to the generosit}'- of 

 Mr. Heron- Allen. 



If any of the forms of Spijnllina are perforate, it is surely 



Sj). vivipara. in which the apparent 

 pores are as a rule so abundantly 

 and uniformly distributed : jei 



^-- Ir^ they are characterized by a strange 



>o CJ /( inconstancy, sometimes so few as 



to be easily counted ; Rhumbler^ 

 for instance, mentions one example 

 in which there are only nine, and 

 in one in my possession there 

 appear to be none. In ^2^. oh- 

 conica the pores are not so obtru- 

 sive as in Sp. vivipara, and out 

 of nine speciniens which I have 

 examined no less than seven show 

 no trace of any : whether they 

 are present in the remaining two 

 I am by no means certain. It 

 would thus appear that the ' pores ' 

 are adventitious, ^ and this inference would seem to find support 



^ The alternative would seem to be that the same species may include 

 both perforate and imperforate individuals. 



Q. J. a. S. No. 307. R 



Fig. 7. — Pseudopores of 

 Spirillina vivipara Wirenherg. 



[Some end within the wall, others 

 extend beyond it, but all end 

 blindly. Upper figure X275; 

 two lower figures X 330.] 



