INTRODUCTION. 



1& 



structure is very curious, and they vary considerably among 

 themselves, some of them being simply conical, others nearly 

 flat, or discoidal, and others more or less spiral. But their 

 principal peculiarity consists in their having a small internal 

 process or plate, commonly named their septum, variously 

 shaped. 



Septa of Limpets. 



The septa of Limpets assume a variety of forms, the prin- 

 cipal of which will be seen in the accompanying engravings. 



The form from which the group derives its generic appella- 

 tion is that of the cup-shaped or Cyathiform species (fig. l7)' 

 In the Crepidulse, or Slipper-Limpets, the septum is flat, 

 reaching across the opening, like the deck of a vessel ; it is 

 then described as transverse (fig. 20). In Calyptrsea Eques- 

 tris, it has two prominent points, and is described as hi-fur- 

 caieJ(fig. 18). In another species, it is a three-sided plate 

 rather spiral at the apex (fig. 19). 



Measurement of Cup and Saucer Limpets. 



The line marked a, p, 1 1, indicates the direction in which 



c 2 



