208 AN INTRODUCTION TO THE STUDY OF FOSSILS 



the body either entirely (in the Aplacophora) or only dorsally 

 (in the Polyplacophora) , and secretes in the former order cal- 

 careous spicules, in the latter a dorsal armor of eight plates. 



Amphineura are marine, in all oceans, 

 with a wide range in depth. They have 

 existed since Ordovician times. All fossil 

 forms are members of the Polyplacophora, 

 — those possessing a dorsal armor (Fig. 87). 



VlTjSrr'ht: class b. pelecypoda 



c»tol''^'Ta''rpenter) Type of class, Veiius mercenaria (Figs. 



from Massachusetts 88— 9 1). 



(but abundant from tt- • n j • i .^i- 



Cape Cod to Flor- V enus mercenarta, called variously the 

 ida). (From Verrill hard or little-neck clam or quahog, is a 

 "^^ ■ warm water form with range of greatest 



abundance along the southern Atlantic coast from the southern 

 side of Cape Cod to Texas. North of Cape Cod it occasionally 

 occurs up to the Gulf of St. Lawrence. It is found most abun- 

 dantly on sandy and muddy flats just below low water mark, 

 but its possible range is great, from high tide line to a depth of 

 over 50 feet. It burrows into the mud to a depth just sufficient 

 to cover the shell. This species has inhabited the Atlantic 

 coast since Miocene time. 



The soft body (Fig. 88, B) is inclosed in a calcareous shell which 

 consists of two halves, the valves, joined by a hinge along the 

 dorsal edge. From one end of the ventral or open edge of the 

 shell protrudes a muscular, tongue-like body, the foot, by means 

 of which the animal may slowly make its way through the sand 

 or mud, in which it lies superficially buried. The presence of 

 the foot indicates that end to be anterior and it is in that direction 

 that the animal moves. From the opposite, the posterior end of 

 the shell, project two tubes, the siphons, united almost to their 

 ends. 



When the valves are opened, each is found to be lined by a 

 thin membrane, the mantle. 



