NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 35 



24. Holderness — On caddis cases in the Leven canal, J.D.B. ; also in the 

 Drift, Beverley Beck, J.D.B. 



Rectification of Districts. 



We take the opportunity of stating that while the system of 

 districts adopted at the commencement of our work has been 

 practically satisfactory, the line separating the Chalk Wolds and 

 Holderness has not been so nearly coincident with the physical 

 features as could be wished. For the future the line of railway 

 from Hull to Driffield and Bridlington will be adopted in place 

 of the river Hull, thus throwing a number of records from the 

 low-lying country on the right bank of that river into the district 

 with which they have most affinity. 



We propose also to separate the records given for district 

 No. 18, or Swaledale, by detaching — by means of the railway 

 from Melmerby to Dalton Junction — the low-lying country of 

 which Thirsk is the centre ; this district we shall describe as 

 1 8a Vale of Mowbray. 



■ -m e e c c e eou-i-- 



Order PULMONOBRANCHIATA. 



Family LIMN^ID^. 



Dr. Gould, in his ' Report on the Invertebrata of Massa- 

 chusetts,' says : ' From the fact of my finding young individuals 

 only in the spring and numerous dead full-grown shells during 

 the late autumn and winter, I presume they arrive at maturity 

 during one season.' This scarcely accords with our experience 

 of this family, as we have generally found them full-grown in 

 summer, and in many cases in spring, and have only found 

 young and immature specimens in autumn. 



The Limnseidse begin to deposit their spawn during the 

 early part of March, and continue throughout the whole of the 



