ARCHER, ON A NEW SPECIES OF COSMARIUM. 253 



the distinctions between certain neighbouring siDecies_, as 

 compared Avith each other, may sometimes depend on a 

 more or less as regards certain common characters, that 

 therefore good and quite distinguishable species do not exist 

 in this genus. Difficult as it may be (to me at least) to define 

 those distinctions, there is at least always a tout ensemble, a 

 general contour, a certain definite combination of character- 

 istics in the same species, whenever met with, which, when 

 they impress themselves on the eye and memory, render the 

 identification by no means difficult. I have never seen even 

 a solitary instance of conjugation except of like admitted 

 species with like. 1 therefore imagine, on the whole, it may 

 be quite possible that certain of the Closteria described by 

 Continental writers, which (if erring, at least, I apprehend, 

 erring on the right side) I have been disposed to think are 

 identical with previously described forms, may be really quite 

 distinct, could one but see the actual specimens. But be 

 this as it may, however I may fail to convey by my descrip- 

 tion the valid distinctions which I conceive to exist between 

 the foregoing Closteria and the nearest allies of each, I have 

 no doubt of them myself, from a careful examination of the 

 living specimens ; and I am glad to say that, having had the 

 honour to submit specimens to M. de Brebisson, my own 

 opinion is coincided in by that high authoritj\ 



Genus — Spirot.enia, Breb. 

 Spirotcenia truncata, sp. nov. 



Specific characters. — Frond cylindrical, but tapering near 

 the truncate ends ; endochrome a single, rather closely wound, 

 spiral band, leaving a minute clear space at each end, often 

 containing one or more free granules. 



Locality. — Featherbed Mountain, co. Dublin, in small 

 pools; rare. 



General description. — Frond minute, five to eight times 

 longer than broad ; cylindrical, but tapering towards the ex- 

 tremities, ends truncate; endochrome at first a single, rather 

 closely wound, spiral band, its revolutions few, frequently 

 afterwards scattered and indistinct in each fully grown frond, 

 leaving at each end a minute, semicircular, clear space, in 

 which there occur one, or perhaps tAvo, free granules ; gela- 

 tinous investment very evident. 



Measurements. — Length of frond, 3-^ ; breadth, -ryTTo of 

 an inch. 



