156 



some of large size and very deeply coloured, others of smaller size, having 

 a delicate silken appearance from the prominent and very neat succession 

 of concentric strise. 



Species. 



1. bifida, Nutt. 3. fragilis, Bory. 5. prolongata, Blain. 



2. exigua, Lam. 4. globosa, Sicainson. 6. umbilicata, D'Orbigny. 



Figure. 



Lanthina globosa. PI. 15. Fig. 80. Shell, showing its light inflated 

 form and violet colour. 



Family 10. NERITACEA. 



Shell ; abruptly transversely convoluted and more or less globose, 

 with the spire short and the last whorl much enlarged. 



The Neritacea present two very distinct groups, which, but for the 

 similarity in the typical structure of their shells, exhibiting a peculiar 

 transverse form of convolution, might be regarded as separate families. 

 The first, comprising the genera Navicetta, Neritina, and Nerita, is an 

 extensive series, not varying materially in size, inhabiting seas and rivers, 

 of which the animal is small, and the shell remarkable for its beautiful 

 varieties of painting. The second group, consisting of the genera Natlca 

 and Sigaretus, the latter of which was until lately associated with Haliotis, 

 is characterized by an animal of much larger proportions, generally more 

 or less enveloping the shell by a thickened ridge and dilated extension of 

 the foot. All the species of this division are of marine habits.* 



Navicella. Nerita. 



Neritina. Natica. 



Sigaretus. 



exserting the end of the long proboscis, which is then seen to be armed with numerous curved 

 hooks ; their eyes are very small and indistinct, and are placed on a short peduncle on the 

 outside of the conical tentacles. If injui'ed or irritated in the water, they eject a very pretty 

 violet-coloured fluid, which appears to be evanescent like iodine. These " schools of sea-snails " 

 comprised about four distinct species sailing among themselves in an indiscriminate manner, be- 

 coming more numerous towards the evening, and entirely disappearing when the breeze sprang 

 up and ruffled the surface of the sea." 



* The genus Neritopsis, hitherto referred to this family, is removed, together with the genus 

 Narica, to the family of the Velutimdae. 



