UNIVALVE SHELLS. 43 



Their surface is highly polished, contributing much to heighten the delicate 

 and glowing tints which are dilf used over some of tlie liner species in an in- 

 finite variety of undulations, clouds, spots, and bands. 



Genus 21.-.CYPR^A. 



Animal a slug ; shell univalve, involute, sub-ovate, smooth, 

 obtuse at each end; aperture effuse at both extremities, liiiear, 

 extending the whole length of the shell, and dentated or toothed 

 on each side. 



Linae divides the genus Cypraea into four families : *mucronate, or point- 

 ed ; **obtuse, and without any manifest spire ; ***umbi!icated, or with a 

 small perforation ; ****marginated. 



The genus Cypraaa consists of beautifully coloured, and very highly polish- 

 ed shells. It is said, that they leave their shells annually, and construct new 

 and larger ones. But we have not been able to ascertain this point as a fact. 



They live in sand at the bottom of the sea ; the animals are provided with 

 a meicbrane, which they throw over their shells, which preserves the fine 

 polish, and prevents other testaceous bodies from fixing on them. The 

 Cypraea have only been found to inhabit the ocean. 



Cypraea Europea. — The European Cypraea. Plate VIII. 

 i\g. 21. With numerous transverse furrows, tumid on the back, 

 pale purple or whitish, and generally with three dark blackish 

 brown spots. Half an inch long. Inhabits the British seas. 



Genus 22.— BULLA. 



Animal a Limax; shell univalve, convolute, and unarmed 

 with teeth", aperture a little straitened, oblong, longitudinal, 

 very entire at the base ; pillar oblique and smooth. 



Bulla Catena.— The Chain Bulla. Plate VIIL fig. 22. 

 (Bullaea Catena, Lamarck.) Ovate, white, pellucid, with a 

 minute chain-like transverse striae ; aperture extremely large, 

 obtuse at the top, with a single volution one-fourth of an inch 

 long. Inhabits the British seas. 



The shells composing the genus Bulla inhabit the ocean, lakes, rivers, 

 and ditches. The marine species are found a few inches under the sand. 

 Some of the species may be frequently taken alive in little pools, between 

 high and low water-mark, in which they are often seen sporting, with a 

 very rapid motion in the sunbeams. 



The animal inhabiting the Bulla is considerably too large for its shell, and 

 cannot contract itself wholly within it, as most other animals which have a 

 testaceous covering can do. 



