102 BRITISH DESMIDIE.E. 



17- C. ammnum (Breb.) ; frond twice as long as broad^ with parallel 

 sides and rounded ends, deeply constricted at the middle, rough 

 M'ith pearly granules. 



Cosmarium amoenum, Brebisson, in lit. cum icons (1846). 



Ambleside, Westmoreland, Mr. Sidebotham. Dolgelley, J. R. 

 Falaise, Brehisson. 



Frond smaller than that of Costnarium margaritiferum,tvf'\CQ as long as broad, 

 deeply constricted at tbe middle, tbe constriction causing a linear notch on 

 each side ; the sides nearly straight and the ends rounded. Segments cam- 

 panulate, rough with crowded, obtuse, papilla-like, pearly granules which give 

 a dentate appearance to the margin. The side Adew is narrower and about 

 thrice as long as broad. 



In form, Cosmarium amoenum resembles the cylindrical species rather than 

 the compressed ones with which it is here classed ; but its smaller size and 

 nearly parallel sides will distinguish it from the other species of the latter, 

 whilst from the former it may be known, ia the front view, by the linear notch 

 which the coastriction produces on each side. 



Length of frond ^-|-g of an inch ; breadth at constriction yy^-Q ; breadth of 

 segment -^^. 



Tab. XVII. fig. 3. a. mature frond ; b. empty frond ; c. side view. 



** Constriction, in the front view, forming a linearnotch on each side ; 

 end view with a lobe or protuberance on each side. 



t Frond rough with pearly granules. 



18. C. biretum {Qxeh.) ', segments compressed, quadrilateral, broadest 

 at the end ; the end margin convex, slightly truncate at the middle j 

 end vievv^ with a lobe on each side. 



Cosmarium biretum, Brebisson, in lit. cum icone (1846). 



Hastings and Bexhill, Sussex, Mr. Jenner. 



Falaise, Brebisson. 



Frond larger than that of any other species m this section, deeply con- 

 stricted at the middle, the constriction forming a linear notch on each side. 

 The segments are compressed, quadrilateral or somewhat hexagonal, nearly 

 twice as broad as long, narrowest at the base and gradually dilated upwards. 

 The end margin, which is the longest, is more convex in Sussex than in 

 French specimens, and becomes angular from being slightly flattened at the 

 middle. 



The empty frond appears punctate rather than granulate. 



An end view is slightly inflated at the middle. The inflation in this and 

 the two following species depends upon a central protuberance of the segment, 

 similar to that observable in some species of Xanthidium, and in this respect 



