240 AN INTRODUCTION TO THE STUDY OF FOSSILS 



tiny shelled individuals. These egg cases are frequent on the 

 beaches in spring and early summer. So small is any one indi- 

 vidual's chance of surviving the various unfavorable conditions 

 and the many enemies surrounding it that it has been estimated 

 that about one in 20,000 of the tiny embryos reaches maturity. 

 The first shell of the larval stage forms, when preserved,- the 

 extreme tip of the adult shell. It is called the protoconch and 

 consists of a single smooth volution with no anterior canal ; it 

 suggests the shell of Natica in appearance (Fig. 104, pr.). This 

 naticoid stage is one that very generally occurs among gastro- 

 pods and one that recalls the characters of early gastropods, such 

 as Straparollina remota of the Lower Cambrian, which is perhaps 

 close to the protogastropod, the earliest gastropod ancestor. 



Just before the animal breaks out from the egg capsule a sec- 

 ond whorl is added w^hich develops tubercles on its shoulder 

 angle and whose lip begins to extend into the anterior canal so 

 characteristic of the adult. The growth of this canal tilts the 

 plane of coiling of succeeding whorls at an angle to the plane of 

 the first whorls. Hence the protoconch has an oblique appear- 

 ance in the adult. 



1. What is the present habitat of Busycon? Its geologic 

 range ? 



2. As it moves, which way does the spire point ? 



3. Describe the respiration of the animal. 



4. What does it eat ? How is this procured and eaten ? 

 How digested ? 



5. Outline briefly the circulation of the blood. 



6. What sense organs does Busycon possess ? Where 

 situated ? 



7. How does the animal move? 



8. How is the body held in the shell ? 



9. What does the word gastropod mean and why applied to 

 this class ? 



10. Sketch {a) shell, looking into aperture and mth spire 

 pointing upwards ; {b) section of shell from tip of spire to 

 siphon. Label whorl, columella, inner and outer lips, spire, old- 

 est portion of shell. 



