2l8 AN INTRODUCTION TO THE STUDY OF FOSSILS 



sharp line separates this new shell growth from the earlier larval 

 shell, — the prodissoconch, which had a smooth, homogeneous 

 structure (compare Fig. loo, pd.). 



During this attached stage the various organs of the body 

 slowly take on adult characteristics. The clam remains at- 

 tached until it has become large enough to make its way into 

 the sand. When it is about q mm. long the byssus disappears 

 and the active foot burrows into the sand, pulling the shell after 

 it in a series of jerks, the siphonal end of the animal being left 

 projecting upw^ard in the burrow. Hereafter it moves but 

 little, occasionally, however, crawling short distances. 



Though the young clams sustain heavy losses before they 

 become able to burrow, the adult animal is one of the hardiest 

 of pelecypods. It is very insensible to temperature changes 

 and to changes in the saltness of the surrounding water, and it 

 can endure' long exposure to the air. There are few natural 

 enemies of the adult clam. Even the starfish and oyster drill 

 are not very destructive. 



They are known to hve four or five years and perhaps in 

 some cases longer. Growth is most active in August, being 

 nearly at a standstill from November i to May i. One indi- 

 vidual with length of an inch when measured was found to 

 have grown | of an inch the following year. Fertilization (in 

 New England) occurs from early June to the middle of August. 



1 . What is the common name of Venus mercenaria ? 



2. Its present range of habitat? Its geologic range ? 



3. Examine the specimens preserved in alcohol : {a) with 

 valves held apart exhibiting relation of mantle to shell, the 

 gills, muscles and foot ; {h) with one valve raised preserving 

 attachment at hinge, showing upon the mantle the following 

 muscles and upon the shell their attachment scars: (i) mantle, 

 (2) siphonal, (3) adductor, and (4) foot. 



4. Examine a dried specimen, sectioned through the two 

 valves from ligament anteriorly, showing the ligament which 

 opens the valves, one of the muscles which closes it, the lines 

 marking the growth of each valve in length and thickness. (The 



