118 DUBLIN UNIVEKSITY 



inflated angles, and by the want of a connecting band in the front view. 

 It resembles more nearly Staurastrum mimes (Kiit%.), an end view of 

 which is figured in Ralfs' monograph, but which has not yet (I believe) 

 been found in Britain ; but that species presents five angles, not three or 

 four, as in this species. I do not think I need contrast it with any other 

 species. The angles in the end view of the quadrangular form are right 

 angles, and the sides straight ; in the triangular form the end view is 

 equilateral and straight-sided, both forms possessing a single awn or 

 acute spine at each angle. The awns are a little longer in the triangular 

 variety than those of the quadrangular. I was fortunate enough to meet 

 with the sporangium of this species (Fig. 13) ; it is spherical and acutely 

 spinous, in fact, very like that of Arthrodesmus incus. That this form 

 is a sort of connecting link, as it were, between Arthrodesmus and Stau- 

 rastrum seems probable, from the not remote likeness in the front view 

 to Arth. incus j as well as from the similarity of the sporangium in each. 

 I would, therefore, venture to put forward the following to serve as a 

 description of this species : — 



Staurastrum O'Mearii (». «.). 



Frond very minute ; segments smooth, ends truncate (in the quad- 

 rangular variety slightly convex) ; central constriction not deep, forming 

 an obtuse angle ; constricted portion very short ; a single awn at each 

 angle ; awns diverging in the front view, acute. 



End view quadrangular or triangular; sides straight;* angles not 

 inflated. 



Sporangium orbicular, spinous; spines at first subulate, afterwards 

 slightly inflated at the base, acute. 



Length of frond of quadrangular variety, -raWth of an inch ; breadth 

 at end (exclusive of spines), t£k ; diameter at isthmus, ^us ; length of 

 spine, T&ns> 



Length of frond of triangular variety, -rrVsth of an inch ; breadth at 

 end (exclusive of spines), ^W ; diameter at isthmus, ?&ns ; length of 

 spine, y^y. 



Diameter of sporangium, without including spines, -r»Vo-th of an inch ; 

 including spines, ^hrth of an inch. 



* I fear the figures (Figs. 9, 11) may convey the idea of the sides being somewhat 

 concave : they are quite straight, and have the angles tipped merely with an awn. 



