THE NAUTILUS. 39 



objectionable, but it makes necessary a nightly journey of about 

 two feet to the nearest growth of algae on which they feed. 

 This activity stretches the girdle downward from the edges of 

 the valves and permits a free play of all the valves so that the 

 mantle deposits its shelly secretions according to the normal 

 habit of the species. The specimens living in the pholad holes, 

 however, apparently never leave them as they are frequently 

 found feeding on the fucus which overhangs them. It protects 

 them from the light, so they have no occasion to move about, 

 and the sand which is washed down into these burrows would 

 make re-entrance almost impossible. A series of these speci- 

 mens shows a gradual change of form. The young specimens 

 are very similar to young specimens from the tidepools, but as 

 they increase in size they become crowded so that the valves 

 press against each other, especially at the posterior end where 

 the valves are bent back across the bottom of the hole. This 

 crowding of the valves upon each other and the crowding of the 

 girdle against the outer edges of the valves so displaces portions 

 of the mantle as to cause the changes noted above. 



Several specimens from each situs were disjointed and a study 

 of the individual valves showed that those from pholad-hole 

 specimens were thicker and had shorter sutural plates and a 

 wider sinus, this last being especially noticeable in the valves 

 from the posterior end. Apparently this change in the sinus is 

 the result of the broadening of the connecting ligaments due to 

 compression by the crowding valves. 



A count of the insertion plates of these disjointed specimens 

 was made and considerable variation noticed. So much, in 

 fact, that more specimens were pulled apart for the express 

 purpose of counting these plates. Representative counts were 

 as follows: 9 slits on the anterior valve, 2-3 on the median 

 valves, and 10 on the posterior. Others show 12, 2-3, 8; 11, 

 2-3; 14, 3-4, 11. Absolutely no difference in this character 

 could be found between specimens from the tidepools and those 

 from the pholad holes. 



On page 64 of vol. xiv of the Manual of Conchology, Dr. 

 Pilsbry says, ' ' Carpenter has given a varietal name to a broad, 

 worn specimen which he thus describes: 



