14 INTRODUCTION. 



is no winding or curvature, but a simple depressed cone, and 

 that the line a, p, divides it into two equal parts. 



The anterior, a, (cut, fig. 10) is known by the interruption 

 of the muscular impression which surrounds the central disc 

 (d.) This interruption of the muscular impression is in the 

 place where the head of the animal lies in the shell. The im- 

 pression itself is caused by the fibrous muscle which attaches 

 the animal to the shell. The apex (a) in Patella, gene- 

 rally leans towards the anterior (a) part of the shell, and away 

 from the posterior (p); and this circumstance has caused some 

 mistakes, because in Emarginula the apex leans towards the 

 posterior; and students, instead of examining the muscular 

 impression, which is the only criterion, have only noticed the 

 direction in which the apex turned, and concluded that to be the 

 anterior, towards which it inclined. The lines or ribs running 

 from the base to the apex of the shell, in the direction r, are 

 called radiating lines ; and those which encircle the cone in the 

 direction c c, from front to back, are very properly described 

 as concentric. The length is measured from front to back in 

 the line e ; the breadth, from side to side, in the line b ; and 

 the depth from the apex to the base. 



Let it be observed that patelliform, or limpet- shaped shells 

 are not all symmetrical; Umbrella, Siphonaria, Ancylus, &c. 

 will form exceptions, of which we have yet to speak. And the 

 learner may also be reminded that the Limpets themselves are 

 not all regular in their form : for as they adhere to rocks and 

 other rough surfaces, and are so little locomotive, in many in- 

 stances they partake of the inequalities of the surface, and 

 conform to its irregularities. This adherence is not effected by 

 any agglutinating power in the animal, nor by any tendinous 

 process like that described above; but simply by means of the 

 foot of the animal acting as a sucker. 



The next variation in symmetrical univalves is to be ob- 



