142 UNIVALVES. 



32. Which are the shells anciently called 

 Purpuras ? 



33. Describe the manner in which the purple 

 dye was obtained and prepared, the different 

 uses to which it was applied, and the circum- 

 stance supposed to have led to its discovery. 



34. What is the generic character of the 

 Trochus, and what its most distinguishing mark ] 



35. Describe a remarkable peculiarity in one 

 of the 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 distin- 

 guished 1 



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 1 



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 

 Haliotis, and what its distinguishing mark ? 



44. What is known of the habits of the animal 

 inhabiting the Haliotis ? 



45. What is the generic character of the Pa- 

 tella, and what its most distinguishing mark 1 



46. Describe the different groups into which 

 the genus Patella is subdivided ? 



