372 Mr. J. Ralfs on the species q/ Desmidium. 



Caccahis 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 o/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 Agardli^s ' Con- 

 spectus criticus Diatomacearum' or in any of om* British works. 

 Its best distinctive character appears to consist in the crenated 

 appearance of the filaments, which is least evident vnD.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 1 have met 

 with I subjoin it : — 



" Desmidium. Corpuscula geminata in longam seriem conjuncta, itaque 

 fihim articulatum constituentia, tubulo communi fiUformi mucoso inclusa." 

 — In Linna?a, 1S33, p. 613. 



Ehrenberg's Desmidiu7n 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, hifidum, acu- 

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

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

 FrustuUa, Ag., is from Fragilaria. 



Ehrenberg defines his genus in the following terms : — 



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

 .^aepe cateniforme." — Die Infus. p. 140. 



