15 108 Proceedings of the Royal Irish Academi). 



uncovered during low-tide, and have not been noted in pools ; thus, though 

 swept with a considerable amount of brackish water on the ebb-tide, its 

 habitat differs considerably from that of the last species. It is not abundant, 

 but has been found, with the exception of the February visit, each time the 

 localities have been searched. 



When young, the plants are more or less orbicular, but soon become 

 deeply divided and very irregular. They occur in small clusters and are of 

 medium size, an average specimen being 10-12 cm. in length ; a few larger 

 fronds were found floating in pools; but it was not quite certain if they 

 were detached from this species or from M. orbicularis. The thallus is usually 

 about 30 ;u thick; and, unless growing in deep shade, is of a very pale colour 

 (of. No. 91). 



The agreement with M. crepidinum is, in deeply-cut angular specimens, 

 fairly close ; but, in others, it is less marked, the fronds being large and 

 irregular. The American specimens in the Kew Herbarium adhere very 

 tightly to paper, which is not the case with any of the Clew Bay material. 

 The lobed fronds appear to separate it from being a marine form of 

 M. orbicularis. 



Enteromorpha paradoxa var, tenuissima (Kiitz.), Batt. 



A few specimens of this beautiful species were found washed ashore at 

 Old Head (August, 1911). Though in poor condition, they are without doubt 

 referable to the plant named E. Hopkirkii by McCalla. The small ohromato- 

 phores, and the monosiphonous ultimate ramulae, separate it from the majority 

 of the richly- branched species; and its extreme delicacy distinguishes it from 

 E. erecta Hook. The plant has suffered much at the hands of systematists, 

 and its position will probably continue uncertain till the genus is thoroughly 

 revised. I have followed Batters (without investigating the point) who has 

 united it with Kiitzing's E. tenuissima, which he regards as a variety of that 

 author's E. paradoxa. 



Ulva Lactuca L. 



For recent work on the biology and ecology of Ulva, and especially its 

 growth in polluted waters, see Beports to the Sewage Commission (Letts and 

 Bichards '11 ; Cotton '11 b). An account of the structure of the basal disk 

 has also been published recently by Miss Delf ('12). 



In the pure waters of Clare Island the plant is one of the most abundant 

 species in the more sheltered rock-pools during summer and autumn. It 

 is frequent on rocks also at Portlea and at Old Head ; but in none of these 

 localities does it attain a great size. Large specimens may be dredged in 



