Vol. 54.] 



FAUNA OF THE SKIDD AW SLATES. 



501 



the stipes are directed slightly 

 but persistently upwards, 

 and the dorsal wall is con- 

 tinuously curved. The stipes 

 are narrowest at their origin, 

 but widen rapidly, the maxi- 

 mum width being attained 

 opposite theca 10. 



In the largest specimens 

 the width at the origin is about 

 ^ inch (-87 mm.), and the 

 maximum width J^^inch (1-58 

 mm.), but average speci- 

 mens do not exceed ^ inch 

 (1-05 mm.) in width^ The 

 maximum width is main- 

 tained till near the distal 

 extremity, where it dimin- 

 ishes owing to the partial 

 development of the latest- 

 formed thecae ; the stipe has 

 thus the appearance of being 

 rounded off. In some forms 

 the thecae appear like those 

 figured for D. constrictus 

 (Hall), but evidence would 

 seem to show that this is 

 merely the result of peculiar 

 preservation, being due to 

 lateral compression of the 

 stipe. 



The thecae are 

 inch (1'58 to 2*1 mm.) long ; 

 they are narrow at their base, 

 but widen towards their 

 apertures, being often at that 

 point nearly twice as wide as 

 at their base. They are about 

 3 times as long as wide. 

 The outer wall of the cell 

 has a slight double curvature, 

 near the base it is slightly 

 concave, near the aperture 

 slightly convex. 



The thecae number twenty- 

 eight to thirty-two in an 

 inch (eleven to thirteen in 

 10 mm.); they are in con- 

 tact for half their length in 

 the proximal part of the 

 stipe : this increases to | or | 



1 



1 6 



to T2 



