18 INTRODUCTION. 



Measurement, of Symmetrical Conical Univalves. 



Jefi/J, 



The width is measured across the aperture, which is the 

 widest part of the shell. The length (I) from the dorsal part 

 (d) of the aperture to the dorsal part of the whorl (d) on the 

 opposite part of the shell. The ventral part of the whorls is 

 that nearest to the axis, and the dorsal that which forms the 

 outline of the figure. 



NON-SYMMETRICAL UNIVALVES. 



These are conical, irregular, spiral, or convolute. The 

 conical form is when there is no enrolment of the apex. 

 Although the Patellae were described as symmetrical, there are 

 several species of Patelliform shells which are not symmetrical. 

 In Umbrella, for instance, the apex is oblique, the shells being 

 placed obliquely on the animal. In the genus Siphonaria, 

 there is a groove on one side, where the brachia or gills of the 

 animal rest. In the genus Ancylus, it will be observed that 

 the apex bends on one side, and the animal is like the Limnsea, 

 which has a spiral shell. The cup and saucer Limpets, or 

 Calyptrsedae, present a group which requires to be described, 

 differently from the symmetrical or true Limpets. Their 

 structure is very curious, and they vary considerably among 



