70 TABLE OF GENERA. 



it occupies ; but when it is short, is sometimes only 

 slightly recurved, as in the Ancylus and Velletia), in 

 order to maintain a similarity of terms for the same 

 thing in these two forms, all the lines or grooves which 

 pass from the apex of the cone to the mouth, and which 

 are caused by some permanent modifications of the 

 edge of the mantle, are called longitudinal or spiral, 

 and all the lines which are parallel to the edge of the 

 mouth of the shell, and which, in fact, are generally 

 marks of its growth, or are caused by some periodi- 

 cal development of the margin of the mantle, are de- 

 signated as concentric or transverse. Thus the striae 

 on the Cyclostoma elegans and Planorbis albus are 

 longitudinal or spiral, and the lamellce on Helix la- 

 mellata and H. aculeata are concentric or transverse. 



But when we speak of the spiral shell as a whole, it 

 is usual to call it short or elongate, according to the 

 length of the imaginary axis on which the whorles are 

 rolled ; and when we speak of the length of the mouth, 

 it extends from the line which forms the front to the 

 hinder edge of the mouth, which, in the Ancylus, occu- 

 pies the whole length of the shell : the breadth is the 

 line which crosses this at a right angle. 



It is equally easy to determine the natural position 

 of the bivalves without the presence of the animal; for 

 the ligament is always placed on the dorsal surface of 

 the animal, and the mouth is placed on that side of 

 the apex of the valve, or umbo, which is before the liga- 

 ment. Consequently, if a bivalve shell is placed on 

 the table, with its hinge-side uppermost, and with the 

 ligament towards the observer, the shell will be in its 

 natural situation, and the sides of the shell will agree 

 with the sides of the observer. 



