The fauna of an inter-tidal mud fiat 



191 



unduly the proportion of the smaller ones, and thus at 10 mm there may tend to be an 

 artificially sharp break in the distribution pattern. Some confirmation of this, for 

 example, was obtained for the distribution of Mytilus in November 1945 (Table XII) 

 when some quadrat counts were also made of small bivalves. Nevertheless a fair 

 indication of the general pattern of the size distribution has been obtained (Tables X, 

 XI, XII, XIII). The growth rates have been based on an examination of modal 

 lengths; the use of growth rings was attempted, but numerous " false rings " inter- 

 fered with the analysis. 



Table XII 

 The size-frequencies o/ Mytilus edulis in North Bay. The figures are averages per m'- 

 based on 6 stations. (Tr. 1 , St. 3; Tr. 2, St. 3; Tr. 3, Sts. 3 and 4; Tr. 4. Sts. 2 and 3) * 



Size 



Groups 



(mm) 



1-3 4-6 7-9 10-12 13-15 16-18 19-2! 22-24 25-27 28-30 31-33 34-36 



July "44 

 Nov. '44 

 July "45 : 

 Nov. '45 I 

 July '46 I 



75 



35 

 21 



11 7 



7 21 



21 — 



168 294 



7 7 



87 

 15 



36 

 14 



0-5 



0-5 



0-5 — 



13 



15 



0-5 

 20 



20 



20 



1 

 16 



1 

 15 



0-5 



0-5 

 11 



Size 



Groups 



{mm) 



37-39 40-42 43-45 46-48 49-51 52-54 55-57 58-60 61-63 64-66 67-69 



July '44 

 Nov. '44 

 July '45 

 Nov. '45 

 Julv '46 



0-5 0-5 — — — — 1 0-5 — — — 



05 _____ — — — — — 



4 4 2 10-5 0-5 0-5 1 0-5 0-5 0-5 



Table XIII 

 The size-frequencies o/Cardium edule in North Bar. The figures are averages per m- 

 based on 6 stations. {Tr. 1, St. 3; Tr. 2, St. 3; Tr. 3, Sts. 3 and A: Tr. 4. Sts. 1 and 2). 

 Note: Up to\0 mm length the groups are 3-5 tnm; thereafter 4 mm groupings are used* 



* Data for November 1943 omitted as sampling methods then not strictly comparable uiih later 

 samples. 



As regards Macoma, almost all individuals were less than 15 mm. The July 1944 

 and July 1945 collections suggest that young bivalves, less than one year old (O-Group), 

 were about 2-4 mm long (Table X). The November figures indicate a growth to about 

 5-6 mm, but the distributions for July in each year also suggest another peak at about 

 8-9 mm. It is suggested therefore that this would represent approximately one year 

 of growth. On this hypothesis there would appear to be little growth from July to the 



