CYPIUEA. 3 



Few of the characters which, in combination, enable us to 

 distinguish species of Oyprcea are sufficiently constant and 

 reliable to be depended upon singly. Within the limits of 

 a species there are often variations, not only in general form 

 and colours and marking, but in the number of teeth, their 

 extension over the base, the production of extremities and 

 thickness of margins, and, in Trivia, the continuation of 

 dorsal ribs and the distinctness of dorsal sulcus. And yet, 

 as a rule, the species are distinguishable at sight without any 

 very great difficulty. 



Some local cause has produced a curious variation in 

 several species found in New Caledonia. The laterally- 

 compressed and terminally-extended variety of G. moneta, 

 described some time since as 0. Barthclemj/i by M. Crasse, 

 has been followed by similar abnormal conditions of C. Ara- 

 bica, C. annulus, G. lynx, and C. stolida, which have been 

 respectively named as new species, C. eglantina, C. Nomne- 

 ensis, C. Caledonica, and C. Crossei. The Sandwich Is- 

 lands also produce a series of varieties of different species, 

 all characterized by a peculiar yellowness in the final glaz- 

 ing of the shell, and some by a thinness and sharpness in 

 the teeth and sides, differing from the normal condition of 

 the species as well known in other localities. 



The greater part of smooth and polished species of Cyprcea 

 have their homes within the tropics, although there are 

 some both north and south. The ribbed species (Trivia) 

 extend very far southwards and northwards, but do not 

 abound in the tropics. 



From the circumstance noticed in many species of Cyprcea 

 — C. tigris being a prominent example — of small shells be- 

 ing found with thickened sides and base, while larger ones 

 are often lighter and apparently less matured, controversies 

 have arisen in past times. Theories about casting or dis- 

 solving, and renewing the shells at different periods of the 

 animal's existence, have been seriously put forth by eminent 

 authorities. These theories have been deemed necessary to 

 account for an apparent, but not a real difficulty. We have 

 only to suppose that the animal ceases to grow at an earlier 

 or later period of its existence. As soon as the animal 

 ceases to grow, and no longer requires to enlarge the 

 volume of its shell, it begins to spend its calcifying powers 

 in thickening the base and outer walls. If this is begun 

 early, the supply, not exhausted in the increase of volume, 



