UNNiEAN ARnANGEMENT. 19 



both arc flat (y)/. 4. /. 4(). Anomia Placenta^. The valves are 

 o'euerallv attached to each other bv a cartilaffe that serves for 

 a hinge; there is seen on tlie oulside of each valve, near tlic 

 cartilag-c, a kind of bkuit point, which is as it were the base 

 of the shell, from whence the increase or growth takes place ; 

 this point is called the beak, in Latin umbo or nates (p/. 3. 

 /. 37. f/) ; the hinge part is called the base of the shell Q)l. 3. 

 f. 26. a): and the opposite part the upper margin QA. 3./. 26. Z>) ; 

 that side of the beaks where the cartilage is found is called 

 the anterior slope (^pl. 3. /. 26. c), and the other side of the 

 beaks the posterior slope Q)l. 3. f. 26. rf). 



The Univalves are said to consist of only one piece, though 

 many species have a smaller piece also, with which the animal, 

 after retiring within his shell, closes the aperture ; this is called 

 the operculum ; but as it is never attached to the shell, but 

 only to the foot of the animal, this part is seldom met with in 

 collections. Most univalve shells are a kind of tube rolled up 

 more or less in a spiral form ; that end where the turns can be 

 perceived is called the spire (/>/. 5. /. 60. o) ; each turn or 

 circle is called a whorl or volution ; the outward volution is 

 called the last whorl, being the one last formed, and is often- 

 times larger than all the preceding ones taken together ; tlie 

 centre or point where the first whorl begins is termed the a])ex 

 (^pl. b.f. 60. e). Linna?us calls that part of the aperture of the 

 shell which is furthest from the spire the base (^pl. 5. /. 60. 6), 

 and the spiral end the/o/e part. The aperture of the shell is 

 called the moiiHi, the edges of which are called lips ; the outer 



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