238 ARCHER.^ OX DESMIDIACE.E. 



Genus — Xanthidium {Ehr.). 

 Xanthidium Smithii (sj). nov.). 



Specific characters : Segments trapezoid ; spines minute, 

 straight, acute, marginal, in four pairs; central projections 

 minute, rounded, tubercle-like. 



Locality : Numerous specimens were met with by me on a 

 slide marked " Wareham, 184-9, W.S., W kindly lent to me by 

 Professor Harvey. 



General Description : Frond minute, in front view about 

 as long as, or very slightly longer than broad ; constriction, 

 a deep, gradually widening notch ; segments about twice as 

 broad as long, trapezoid, lower margin somewhat convex, 

 sides narrowing upwards and straight, ends broad and straight; 

 spines short, minute, straight, acute, marginal, geminate, a 

 pair placed on each of the four angles ; in side view, segments 

 about as broad as long, with rounded sides, ends truncate, 

 each upper angle furnished with a minute spine, beneath each 

 of which, about halfway doAvn the segment, there occurs 

 another spine, all the spines divergent ; end view sub-elliptic 

 or broadly fusiform, ends truncato-convex, furnished with 

 three spines, the spines projecting, divergent, none at the 

 sides; the central projection from each front surface a 

 minute, rounded, smooth tubercle, apparent always in end 

 view ; in the side view the tubercles are sometimes concealed 

 by the projecting divergent central spines, while in the front 

 view they are hidden by the contained endochrome. Spo- 

 rangium unknown. 



Measurements : Length of frond, ttV s > breadth, i-Jj^ of 

 an inch. 



Plate XL — Fig. 10, front view ; Fig. 11, side view ; Fig. 12, 

 end view. 



Affinities : This interesting little species seems to be allied 

 on the one hand to Xanthidium fasciculatum | Ehr.), and on 

 the other to Arthrodesmus octocornis (Ehr., llass., Breb.)= 

 Xanthidium octocorne (Kalfs), var. ft, but is perfectly and, 1 

 believe, unmistakably distinct from cither. With the former 

 it agrees in the spines being subulate, marginal, anil in pairs, 

 and in the central projection being not granulate : but it 

 differs in its far more minute size, and in its segments being 

 four-sided, not reniform or sub-hexagonal, and in its spines 

 being short, and minute and straiglit, not elongate and 

 usually curved, and in its constriction not forming a linear 

 notch. With the other species named (var. /3) this form 



