STAURASTRUM. 159 



y-cwm-ffynon, Carnarvonshire ! Dolgelly, Merioneth 

 (Rolfs}. In the plankton ! 



SCOTLAND.- -Ross, Inverness, Aberdeen, Kincardine 

 and Perth (Roy & Biss.}. 



IRELAND.- -Donegal ! Mayo ! Galway ! Kerry ! Dub- 

 lin and Wicklow (Arch.}. Down ! 



Geogr. Distribution- -France. Belgium. Germany. 

 Switzerland. Galicia in Austria. Hungary. Italy. Nor- 

 way. Sweden. Denmark. Bornholm. Finland. Poland. 

 S. Russia. Japan. Australia. Azores. United States. 

 N. W. Canada. Brazil. 



Few species exhibit so much variation as St. vestitum whilst 

 retaining at the same time their distinctive features. The main 

 diagnostic character of the species is the possession of the pair 

 of furcate spines in the middle of the lateral margins of the 

 vertical view. These spines themselves are subject to much 

 variation, being sometimes simple aculei, at other times furcate 

 to their base, or more rarely, doubly furcate. The general plan 

 of the arrangement of the spines and of the emarginate verrucas 

 is precisely like that of St. aculeatum. The angles of St. vestitum, 

 which possess 3 well-marked divergent spines at their apices, 

 are more produced than those of St. aculeatum, and as a rule, the 

 two median spines of the dorsal series become converted into 

 emarginate warts. Of the lateral series of spines, which are 

 such a marked feature of St. aculeatum. either the two median 

 spines only remain in St. vestitum, or they are much more 

 prominent than the rest. These are the characteristic furcate 

 spines mentioned above. Xo matter how variable the lateral 

 series of spines may be (including the furcate ones), those of the 

 dorsal series are always disposed on the same plan. The front view 

 of typical St. vestitum resembles very closely that of some forms 

 of St. aculeatum, but in the majority of specimens of the former 

 species, the angles are produced into processes of varying length. 

 In some of these the processes are very long, making the total 

 width of the cell as much as 90 /n ; and in others they are very 

 short, so that a breadth of about 46 /LI only is attained. Archer 

 has reported the occurrence of a biradiate form of St. vestitum. 



Var. subanatinum W. & G. S. West. (PL CLIII, fig. 5.) 



St. vestitum var. subanatinum West & G. 8. West, Alg. N. Ireland, 1902, 

 p. 54, t. 1, f. 28. 



