BRITISH DESMIDIE^. 119 



Frond smooth, minute, deeply constricted at tlie middle. The fronds are 

 truncated at the ends and have a mucro at each angle ; a distinct neck at the 

 constriction is sometimes present, sometimes wanting. In a. the segments 

 are semilunate and the spines usually directed outwards. The variety l3. is 

 generally larger than a ; its segments are nearly equal in length and breadth, 

 and being rather suddenly rounded oft" at the constriction, present a gibbous- 

 Uke appearance on each side ; the spines in /j. are stouter, and those of one 

 segment slightly converge with those of the other. This form is considered 

 a distinct species by INIr. Jenner, who thinks that its end view is sometimes 

 triangvdar, and that it unites Arthrodesvms with Staurastrum. 



The orbicular sporangia have been gathered in Sussex by Mr. Jenner, and 

 I have met with them at Dolgelley and Penzance ; they have subulate spines 

 like those of Staurastrum dejectum, which they resemble, except in their 

 smaller size. 



This species is variable both in the form of its segments and in the direction 

 of its spines ; the ends however are always truncate ; a character which aff"ords 

 a good distinction between it and A. convergens. 



Euastrum Incus of Kiitzing's ' Phycologia Germanica' {ArthroJesmus mi- 

 nutus, Bre'b. MS.) appears, from a drawing which he sent me, to differ merely 

 in the smaller size and in its parallel spines. I fear it is scarcely distinct. 



Arthrodesmus Incus, in the front view, has nuich the appearance of some 

 states of Staurastrum dejectum ; it is nevertheless usually smaller, and its 

 ends are more decidedly truncate. 



Length of frond (Tab. XX. fig. 4. a.) y^^^ of an inch; breadth yoV o ; 

 length of frond (fig. 4. e.) from -^^^ to ysVt ' breadth, not including spines, 

 from j-gJg-g- to YT2^ ; including spines, from -g^ to y-i^ ; breadth at con- 

 striction from g-gVs- to 3T2T '■> <liameter of sporangium, without spines, y2'^To J 

 including spines, y-g-3. 



Length of frond of var. /3. -^ ; breadth, not including spines, ttS^5 

 incluchng spines, -— -j- ; breadth at constriction -oyVf 



Tab. XX. fig. 4. a, e. fronds with endochrome ; b. empty frond ; 0, h. end 

 views ; d. frond dividing ; i. fronds conjugating ; k, I. sporangia ; /. frond of 

 var. ft. with endochrome ; g. empty frond. 



11. STAURASTRUM, M^yew. 



Frond simple, constricted at the ttiiddle ; end view angular, or cir- 

 cular with a lobato-radiate margin, or, rarely, compressed with a 

 process at each extremity. 



Frond mostly minute, simple, more or less constricted at the 

 middle, and thus forming two segments, which are often somewhat 

 twisted, generally broader than long, and in many species elongated 

 laterally into a process, so that the constriction on each side is a 

 roundish or angular sinus ; in other respects the front view shows 

 the segments quite entire. 



