58 Procet'i/inr/s of ihr Royal Irish Academy. 



or Jauuaiy ; but as it was present in December, it is likely that the numbers 

 were too small to be represented in the counts. The maximum was reached in 

 June (9,000 in aU catches). Other periods of maxima have been recorded 

 between May and August ; and in Some places — Xorth American lakes — it 

 develops exceptionally in winter. In other lakes it appears to have two 

 maxima, as in the Acheu See, where there is a spring maximum in March and 

 April, and a chief maximum in August. 



Wesenberg Lund records the species for aU the lakes investigated in 

 Denmark. It was present throughout the year, and reached its maximum 

 between May and August. Apstein records the species from one of the Baltic 

 lakes — Ploner See — where it reaches its maximum in August, and is absent 

 during aSTovember, December, and January. It seems to be more common in 

 Scotland than with us; for Murray and PuUar record the collections of 

 plankton as sometimes brick-red in colour, through Notholca longlsp-ina. 

 Kofoid notes N. longisjiina only once in the plankton of the Illinois Ei\'er, 

 January, 1895. 



Triarthra. 



Triarfhra lo-ngiscta Ehr. — Occurs iu Lough Neagh in fair quantities, being, 

 after Anurea coclilearis and Polyarthra platyptera, the next important Eotifer 

 in frequency. It appears about the end of March, and increases slowly in 

 numbers at first, attaining its maximum in July (39,000 in aU the catches). 

 After this the numbers drop away suddenly, and in Xovember it disappears 

 completely. 



"Wesenberg Lund records the species from all the lakes investigated in 

 Denmark, where it is perennial, the sexual period occurring in May. Apstein 

 does not record T. Imigiseta fi-om the Dobersdorfer See ; but m the Ploner See 

 it is present. The maximum occurs there in August. In the Scottish 

 lochs, Murray states that T. longiseta is comparatively rare, and is locall}- 

 distributed. The species is cosmopolitan. Kofoid records T. lungiseta from 

 the Illinois Eiver, where it occurs throughout the year. Their numbers are 

 more frequent from May to October, and the maximum is reached in 



September. 



Notholca. 



Notholca striata Ehr. — A less common constituent of the plankton of Lough 

 Neagh. It is absent from the end of April to SeptemlDer. In October and 

 the following months it occurs in small numbers, and it reaches its maximum 

 in February and March (22,000 in the combined catches). 



Wesenberg Lund records N. striata from most of the lakes of Denmark, 

 where it attains its maximum in the period December to April. It was never 

 observed iu summer. Apstein records X. striata from the Ploner See in 



