AKC11EK — NEW SPECIES OF COSMAEIUM AND ARTHRODESMUS. 69 



many species, and it has been figured and described by several observers ; 

 it is simply a monstrosity.* 



Supposing the question of the genus determined, but quite irre- 

 spective of it, this new form is quite distinct as a species, and cannot 

 be mistaken for any othe:-. Its size alone would almost distinguish it 

 from any other Desmid ; for, notwithstanding that the measurements of 

 Cosmarium tinctum, and of the Cosmarium I have just described above, 

 ( C. pygmceum) are not much greater, the present species probably enjoys 

 the distinction, so far as I am aware, of being the very smallest of the 

 free species of the family, for the concavities of the form under consi- 

 deration render its actual bulk or volume very appreciably less. It is, 

 therefore, considerably smaller than any other Arthrodesmus. Prom 

 Arthrodesmus bifidus (Breb.), its straight, not curved and bicuspidate, seg- 

 ments and its four terminal mucrones in end view, abundantly distinguish 

 it. From Arthrodesmus octocomis it is distinguished by its much less 

 elongate and less acute lateral extremities, which in the latter are, as 

 well as the upper angles, prolonged into single or double spines, all di- 

 vergent in front view — while in the new form the four external spines 

 are well seen only in the edge views, and are not in a single series. 

 Prom A. octocomis, var. /3., as well as from Xanthidium Smithii, (mihi), 

 this species is distinguished by its not having a double spine at each 

 angle, and from the latter, also, by the want of the central protube- 

 rances. But, besides these more tangible and prominent diagnostic dis- 

 tinctions, I need hardly remind those who are thoroughly acquainted 

 with this elegant family of Algae, that each of them — these new ones 

 included — presents to the familiarized eye a special tout ensemble of its 

 own, cUificult perhaps to define in words, but, in my mind, not the less 

 actual, constant, distinctive, and characteristic. 



Note. — I have in the foregoing made use of, and shall for the future 

 employ, the more apt term " Zygospore," as suggested by Professor De 

 Bary in his work " Untersuchungen fiber die Pamilie der Conjugaten," 

 in preference to the seemingly inappropriate and inaccurate term " Spor- 

 angium" of most other authors. 



* See Mrs. H. Thomas, " Quarterly Journal of Microscopical Science," vol. iii., Pl.V., 

 Figs. 17, 18 ; Hofmeister, "Bericht der K. Sachs. Ges. der Wissenscli. zu Leipzig," 1857, 

 Hef I. ; DeBrebisson, " Liste des Desmidiees observees en Basse-Norinandie," Tab. I., 

 Fig.15; DeBary, " Untersuchungen iiber die Fam. d. Conjugaten," p. 47, T.VL, Fig. 53; 

 W. Archer, "Proc. Nat. Hist. Soc. Dublin," vol. ii., p. 207, Figs. 10-15; also "Nat. 

 Hist. Review," O. S., vol. vi., p. 469, PI. XXXIII., Figs. 10-15 ; and again " Proc, 

 Nat. Hist. Soc. Dublin," vol. iii., p. 37, PI. I., Fig. 7 ; and " Nat. Hist. Review," 0. S., 

 vol. vii., p. 391, PI XIII., Fig. 7. 



