INTRODUCTION. 



19 



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 variously shaped, commonly named their 

 septum. 



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. 17). 

 In the Crepidulee, 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 Calyptraea Eques- 

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

 cated (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, I I, indicates the direction in which 

 c 2 



