No. 526] VARIATIONS IN UROSALPINX 



589 



more, the analysis of the 1899 shells indicates that a 

 proper comparison of place-modes of variability could 

 be made only on material of the same relative age, which 

 presumably could be approximated best by collecting 

 the shells in neighboring localities at the same time or 

 in any one locality at the same time in successive years. 

 Consequently lots of 1,000 shells from each of the four 

 Woods Hole localities mentioned in Table V were col- 

 lected during the first week of August, with some omis- 

 sions, for several years. These data are assembled in 

 Tables IX and X. 



In Table IX it is made apparent that, when the time 

 element is reduced to a minimum by comparing only 

 August 5 shells of various years, the shells of Buz- 

 zard's Bay (West Shore and Penzance Point) fall into 

 a group distinct from those of Vineyard Sound (Nob- 

 ska Point and Barnacle Beach), at least so far as the A. 

 M. is concerned. The A. M. of 11,476 Buzzard's Bay — 

 August 5 shells is 61.330 while the A. M. for 14,503 Vine- 

 yard Sound— August 5 shells, is 64.303. In no individ- 

 ual lot of Buzzard's Bay shells does the A. M. reach as 

 high as the Vineyard Sound average and in no one lot 

 of the Vineyard Sound shells does the A. M. fall as low 

 as the Buzzard's Bay average of 61.330. 



The standard deviation of the August shells shows 

 no decided grouping with reference to Buzzard's Bay 

 and Vineyard Sound, although those from the latter lo- 

 cality show a slightly higher total average which is 

 probably quite without significance. In general, then, it 

 may be said that during the first week of August the 

 Buzzard's Bay shells show a lower ratio of greatest 

 shell-aperture to height (and consequently may be re- 

 garded as rather more advanced in their life cycle) than 

 those of Vineyard Sound, but that they present no sig- 

 nificant difference in variability. 



In each of the two general localities which were more 

 exposed to the open water and the beat of the waves, 

 viz., Nobska Point and Penzance Point, is the variabil- 



