203 PfiOF. vr. J. soLLAS ox SACCAiiiiiyA caetert. [vol. Ixxvii. 



bluish by reflected llgbt. The granules for the greater part shai-e 

 in a common orientation with their optic axes tangential to the 

 wall, both in the embryonal and in succeeding chambers. 



In large specimens of a JliUola from Lyme Eegis the granula- 

 tion is much coarser, and traversed by scarcely visible clearer 

 streaks. Between crossed nicols these streaks are represented by 

 exti-emely fine stria tions, very close and numerous, alternately light 

 and dark when inclined at 4-5" to the angle of general extinction, 

 and follo\ving the com-se of minute transverse ribs which i-un round 

 the exterior of the shell. As these striations approach the line 

 along which the Avail of the chamber unites with that of an 

 adjacent chamber, they coalesce to form a narrow continuous band. 

 The optic axes of those stiiations which are illuminated when 

 inclined at J:5 " to the angle of extinction are mostly transverse to 

 the sti'iations. 



In Fenerojylis (JP. pertiisiis)^ the structure is in general finely 

 granular. The primordial chamber gives a dark cross between 

 crossed nicols. with the optical sisrn positive : the optic axes must 

 therefore lie tangentially. The ribs show straight extinction, and on 

 rotation through -^o^ from the position of extinction are at maximmn 

 illumination : but the furrows between them show no change, 

 remaining dark at all angles. Probably the wall below the furrows 

 is too thin to affect the light appreciably. Each rib extmguishes 

 as a whole, and when on rotation it restores the light it is uniformly 

 illuminated from end to end, the granular apj^earance so obvious in 

 ordinaiy light being then almost abolished. This is strongly 

 susrgestive of a continuous crystalhne sti"ucture. The direction of 

 the optic axis seems to be subject to no law. In some specimens 

 it is parallel to nearly every rib, in others on the contrary transverse. 

 In one and the same specimen it may be parallel in some zones and 

 transverse in others : even in the same zone it may be parallel in 

 some ribs and transverse in the rest ; and again in the same rib it 

 may be parallel over one half of the lenirth and transverse over the 

 other. 



The septal planes are complex, usually presenting three layers — 

 a middle with the optic axis parallel to the surface and two super- 

 ficial layers with the optic axis transverse, or the direction of the 

 optic axes may be reversed. 



The structure of Cornuspira (C. cari'/iafa) is also finely erranular. 

 The primordial chamber and the immediately sm-rounding whorls, 

 owing to their thin walls and the absence of involution, can he 

 examine:! under fairly hiarh powers without any preparation beyond 

 mounting in balsam. Between crossed nicols they give a well- 

 marked cross, extending from the primordial chamber outwards. 

 The optic sign, observed in four specimens, is negative, and thus, as 

 an exception to the general rule, the optic axes of the crystalline 



- Here I desire to express my obligation to my friend. Mr. E. Heron- AHen, 

 F.E.S., who has lieli)ed me in manr ways, especially by the gift of rich material 

 for stady. 



