DesmidiecB-l iNFrsoRiAL animalcules. 259 



sections of flints, and also fossil occasionally in earth. The reference 

 of these fossil organic remains to this genus is not supported by any 

 acciu-ate examination, and not even by outward characters ; their 

 general characters are as those of spores of DesmidiecB and of other 

 microscopic Algae. 



The number of these so-called fossil Xanthidia has been much 

 added to by other observers; we append those given by Ehi'enberg. 

 XANTHiDitrir (?) ramosum. — Lorica globose, spinous; spines trifid or 

 branched at their extremities ; they vary in number from 6 to 20, and 

 singly or in pau's (P. 12. f. 511 and 515.) Found in flint. Most 

 probably sporangia. Ekrenberg thinks its true place is the genus 

 Peridinium, as traces of a transverse groove have been observed. 

 Diameter 1-11 50th to 1 -280th. 



X. crassipes. — Lorica globose ; large. There are two varieties ; in 

 one the thick blunt spines appear distinct, in the other they appear 

 as a fringe around the lorica. Found in flint. Diameter l-280th. 

 X. penicillatiim. — Fossil in earth from the Jura. 

 X. pilosum. — Fossil in earth from the Jura. 



X. (?) tuliferum. — Corpuscles globose, single and in pairs, aculeate ; 

 spines in the form of tubes, dilated and dentate at the apex. Diameter 

 l-400th. 



X. hulbosum. — Corpuscles globose, single or binate, aculeate ; 

 aculei attenuate, forked at the apex, bulbous at the base. Diameter 

 l-400th. 



Genus Aktheodesitcts. — Frond simple, compressed, constricted at 

 the middle ; segments smooth, entire, with a single spine on each 

 side. The sporangia spinous. Mr. Ralfs says : — " Where the plants 

 should be placed, to the reception of which I have restricted this 

 genus, has been left in much imcertainty. Ehrenberg, making no 

 distinction between constiicted and binate cells, has associated them 

 with others belonging to Scenedesmus, to form his Arthrodesmus. Mr 

 Jenner considers them to belong to Stmirastnim; and he beKeves he 

 has met with one species, some specimens of which were compressed, 

 whilst others had three angles in an end view ; whilst it must be 

 further allowed, that in the front view the resemblance to some 

 species of Stcmrastrum is veiy close. 



Although Mr. Ealfs has employed the name Arthrodesmus to 



