1882.] 



On the Formation of Eipplemark. 



13 



previous day. Two of them were nearly the same size and showed 

 several marks of interrupted growth ; one of the marks was quite 

 unmistakeable, when the shells were about 1^ inches long. I 

 measured each of the convex valves of these pectens independently, 

 recording the size of each when the interruptions to growth occurred. 

 The lengths of each were as follows, measured in inches and six- 

 teenths : — 



No. 1. 



Growth checked when the length of shell was l T 7 g- 



9 1 o 



55 55 55 ^16 



°2 



Oil 



55 ° 1 6 



4 



Total length 4^ 



No. 2. 



H 



2-!* 



H 



n 



Ql 3 

 4 



^1 6 

 4_7_ 



The interest in these pectens lies in the correspondence of their lines 

 of arrested growth, which have clearly been caused by some external 

 agent common to both shells, and are not due to any idiosyncracy. 

 Taken separately both shells are inferior as examples of arrested 

 growth to the specimen of Pecten rnaximus figured by Mr. Jeffreys on 

 Plate XXV of his " British Conchology." 



On the 3rd of March I received the parcel of shells referred to in 

 Hay den's letter. They were taken whilst the crew of the " Pelican " 

 were pursuing their ordinary avocation of fishing, and their collection 

 was spread over a considerable time. The exact locality whence came 

 each shell cannot of course be specified, but it so happens that with 

 the shells I received fragments of three of the Channel stones, which 

 the crew of the " Pelican " have been in the habit of sending me for 

 some years past. These stones give us a clue as to the depth of 

 water where the vessel had been fishing. One stone was taken 18 or 

 20 miles S.S.E. of the Start, another 20 miles S. of the Eddystone, 

 and the third 15 miles S.E. of the Start. The depth of water at the 

 places indicated is (according to the chart) 38, 41, and 36 fathoms 

 respectively, giving an average of over 38 fathoms. The majority of 

 these shells bear on them marks of arrested growth, not, as a rule, so 

 decided as those from shallower water, but in many cases quite un- 

 mistakeable. Some details of the collection are given in the follow- 

 ing table : — 



