242 DESCBIPTION OF {Tolycjastrica. 



DESMiDrtm aeuleatum (Ehr.) = Staunistrum aculeatum (Ealfs.) 

 D. apiculosum (Ehx.) = Staurastrum muricatum (Ralfs.) 

 In a paper published by Ehrenberg in 1840, we have found the 

 following described as species of Lesmidium, but the characters 

 detailed, lead us to the opinion that they really belong to Staurastrum. 

 One of them, indeed, called Besmidium ramosum, has, we observe, 

 been identified by Mr. E.alfs as equivalent to his Stav/rastrwn 

 sjjongiosum. The others are : — 



D. divergens (Ehr.) — Angles of the sides aculeate, recurved on 

 one side, corpuscles binate, divergent ; semilunar on the dorsum ; 

 surface smooth. Diameter l-1150th. If a representative of anew 

 genus ? 



D. tridens. — ^Angles on the sides acute, long, rostrate, tridentate at 

 the apex ; fusiform dorsally ; the entire surface of the centre of the 

 disc armed with spines. Diameter 1 -480th. Is allied to D. hexa- 

 ceros (Ehr.) 



Genus Aptogonum (Ralfs.) — " Eilament elongated, triangular or 

 plane ; joints bicrenate at the margins ; an oval foramen between 

 the joints. Endochi-ome bipartite. — It is closely allied to Desmidmm, 

 but the large oval foramen between the joints is so remarkable a 

 character, that I must concur with Ehrenberg in placing it in a 

 separate genus. As, however, Ehrenberg' s name, Odontella, had pre- 

 viously been given by Agardh to some DiatomacecB, it became 

 necessary to find another name. Elu-enberg included SpJuerotosma 

 in his Odontella, but the present genus is essentially distinct from 

 Splioerozosma ; the latter has the joints incised or sinuated, and 

 gland-like processes at their junctions, and it is merely by the inter- 

 position of these processes that the joints are now and then slightly 

 separated ; in this genus, on the contrary, the joints have two pro- 

 minent teeth at the margins or angles, and the foramen results from 

 the excavation or concavity of the joint itself, and not from the 

 presence of glands. 



A. Besmidium = Odontella Besmidium (Ehr.) — Joints in the front 

 view quadrangular, broader than long; crenatures distinct. lS.o 

 sheath. 



Var. (a.) — Filaments triangular, regularly twisted; crenatures 

 rounded. 



