522 Transactions of the Society. 



beyond this. With central light the transverse sections appear 

 homogeneous, and flat on both sides. In examining the sections 

 I could not trace differences in the membrane thickness ; nor did I 

 observe projections or continuations with one exception near the 

 edge of the valve outside the wing, and this occurs pretty constantly 

 and may stand in connection with the attachment of the girdle-band. 

 On this matter I cannot give more definite explanations. If we exa- 

 mine the cast in view of these facts, the wave shape of the surface is 

 thereby substantiated ; it follows that the fluid collodion must have 

 entered into the tube-system of the wings, and in pulling off the 

 valve there must have been left behind contracted tubes. In 

 reahty, not far from the edge, such protuberances of collodion are 

 seen at regular intervals. The cast shows absolutely nothing of 

 the transverse striae however oblique may be the illumination with 

 which it is examined. From the above we infer, with regard to 

 the finer sculpture, that midrib and transverse rib are both wall- 

 thickenings of which the transverse strise have probably been 

 produced by the cylindrical hollow spaces within the membrane. 

 Small hollows of these cylinders then suggest an appearance of 

 longitudinal lines, and the condition is similar to the transition of the 

 simply striped Pinnularia to the pointed striped Navicula. This 

 lesser definition remains obscure. For microscopists these investi- 

 gations about Surirella sculpture are of some importance, since they 

 explain various peculiarities of S. gemma which may be looked upon 

 as similar to 8. Useriata. At first sight the longitudinal section, 

 fig. 19, teaches us that it really is no brilliant performance for an 

 objective when it shows the much-spoken-of longitudinal striae every- 

 where at the same time. All that is proved is that the objective 

 possesses the power of showing at the same not only striae which 

 are within the focus but others which are beyond. This can be 

 easily obtained with bright sunhght, but with ordinary day- 

 light an objective should only show clearly either the striae on the 

 elevations or those on the depressions, Secondly, the transverse 

 section near the end, fig. 14, estabhshes the fact that in order to see 

 both striae at the same time it is best to examine the end portions of 

 a valve under an obliquity of illumination of 45° to both directions. 

 Here the surface of the valve is smoother. Altogether Surirella, 

 on account of its uneven surface, is a very unsatisfactory test-object. 



( To be continued.') 



