372 Mr. J. Ralfs on the species of Desmidium. 



Caccabis Bonhami. Grayish isabella-colour, numerously 

 banded and vermiculated with dusky ; lower part of the back 

 and tail-coverts slightly speckled with black ; front and line 

 above the eyes black, another line through the eyes white ; 

 the throat and sides of the neck pale blue, with some of the 

 feathers of the latter variegated with white ; breast plain isa- 

 bella-colour ; feathers of the sides varied with blotches of ru- 

 fous isabella and white margined with black ; abdomen nearly 

 white ; tail pale cinnamon, with the ends paler and vermicu- 

 lated with black. Bill and legs yellow. 



Length 8^ inches. 



Female paler, entirely banded and vermiculated with dusky; 

 sides and abdomen varied with white, which colour predomi- 

 nates on the throat. It is from the mountains of Persia, and 

 has much the appearance of the P. Heyii, Temm., to which it 

 also approaches in size. 



LIII. — Remarks on the species of Desmidium. By John Ralfs, 

 Esq., M.R.C.S., Penzance. 



[With a Plate.] 

 Desmidium, Ag. 



Filaments fragile, jointed, angular, with the angles of each joint 

 bicrenate, or cylindrical with a circular groove which produces 

 a crenate appearance, finally separating into single joints or 

 frustules. 



This natural genus is not well defined either in Agardlr's ' Con- 

 spectus criticus Diatomacearum , or in any of our British works. 

 Its best distinctive character appears to consist in the crenated 

 appearance of the filaments, which is least evident mD.mucosum*. 

 The filaments are of a pale green colour, simple, fragile, short, 



* Since this paper was written I have had an opportunity of seeing Kiit- 

 zing's ' Synopsis Diatomearum,' and as his character is the best I have met 

 with I subjoin it : — 



" Desmidium. Corpuscula geminata in longam seriem conjuncta, itaque 

 filum articulatum constituentia, tubulo communi filiformi mucoso inclusa." 

 —In Linnaea, 1833, p. 613. 



Ehrenberg's Desmidium differs widely from that of botanists. D. Swartzii 

 is the only species common to both, in consequence of his having made tri- 

 angular frustules one of the characters of his genus. His Desmidium, be- 

 sides D. Swartzii, contains five species, orbicular e, hexaceros, bifidum, acu- 

 leatum and apiculosum, all found in England. But these, not being united 

 into a filament, are distinguished from the genus as above described, just as 

 Frustulia, Ag., is from Fragilaria. 



Ehrenberg defines his genus in the following terms : — 



" Desmidium animal liberum, lorica simplici, univalvi, triquetra inclusum, 

 saepe cateniforme," — Die Infus. p. 140. 



