38 



THE NAUTILUS. 



tiflepools contained many old pholad holes and in these were 

 found tlie form for which I was searching. Twenty specimens 

 of various sizes were collected from as many pholad holes. In 

 the larger specimens the foot had become so greatly enlarged to 

 fit the concave bottoms of the holes that it was impossible for 

 them to curl up in the usual manner. Some of these specimens 

 were so badly eroded by the sand and gravel which wash in and 

 out of the holes that the anterior valve was reduced to two 

 thirds of its normal height. 



In color pattern, sculpture, and mantle characters these 

 specimens were identical with those from the tidepools, and, as 

 will be seen by referring to the table of measurements, the 

 smaller specimens approach quite closely to the proportions of 

 the typical or tidepool forms. 



Typical or Tidepool. Forms. 



(Lot 1) 



Lon. 

 99 

 83 

 83 

 62 

 59 

 38 



Lat. 

 35 

 33 

 30 

 22 

 23 

 14 



Index. 

 2.52 

 2.61 

 2.76 

 2.80 

 2.56 

 2.71 



Div. 

 130O 

 130O 

 1250 

 125° 

 J 30° 

 130O 



Specimens From Pholad Holes. 



(Lot 2) 



Lon. 

 65 

 63 

 62 

 60 

 56 

 42 



Lat. 

 33 

 32 

 27 

 30 

 29 

 17 



Index. 

 1.97 

 1.97 

 2.29 

 2.00 

 1.93 

 2.47 



Div. 



135^ 

 155° 

 140O 

 130O 

 130O 

 130O 



In brief, Lot 1 shows an angle of divergence constant at 125'* 

 to 130°, where Lot 2 shows an angle varying from 130° to 156°, 

 and a proportion of length to width 2.51 to 2.80 as against a 

 proportion ranging from 2.47 to 1.93. 



The noticeable differences to the eye are first, the narrow and 

 sharply raised lateral areas, and second, the shape of the pos- 

 terior edge of the median valves. In the tidepool specimens 

 the posterior or exposed edge of each valve is a straight line, 

 while in specimens from the pholad holes this line becomes a 

 double convex curve, the most posterior portion of the valves 

 being about midway between the beaks and the girdle. 



These differences seem to be explained by the following facts. 

 In collecting, the tidepool specimens are usually found on the 

 under side of large rocks and well back from the edge. This 

 situs protects them from the light which they evidently find 



