144 UNIVALVES. 



32. Which are the shells anciently called Pur- 

 puree? 



33. Describe the manner in which the purple 

 dye was obtained and prepared, the different uses 

 to which it was applied, and the circumstance 

 supposed to have led to its discovery. 



34. What is the generic character of the Tro- 

 chus, and what its most distinguishing mark ? 



35. Describe a remarkable peculiarity in one 

 species of Trochus. 



36. What is the generic character of Turbo, 

 and its distinguishing mark ? 



37. Which species of Turbo is remarkable, 

 and on what account ? 



38. What is the generic character of Helix, 

 and by what mark is it particularly distinguished ? 



39. What variety of form is displayed in the 

 Helices? 



40. Detail the interesting facts connected with 

 the natural history of the Helix. 



41. How does the Helix lanthina display the 

 compensatory providence of God ? 



42. What is the generic character of the Ne- 

 rita, and by what mark is it readily known ? 



43. What is the generic character of the ani- 

 mal inhabiting the Haliotis ? 



44. What is known of the habits of the Hali- 

 otis, and what is its distinguishing mark ? 



45. What is the generic character of the 

 Patella, and what its most distinguishing mark ? 



