and £18 respectively, and, at the Vernède sale, _Ancillaria Vernèdei_ was bought for £6: 10s., and _Voluta piperata_ for £7: 10s. A unique specimen of a recent _Pleurotomaria_ (_quoyana_ F. and B.) was purchased by Miss de Burgh in 1873 for 25 guineas, and another species of the same genus (_adansoniana_ Cr. and F.), of extraordinary size and beauty, is now offered for sale for about £100. Bivalves have never fetched quite such high prices as univalves, but some of the favourite and showy genera have gone near to rival them. On 22nd June 1869, at Stevens’, _Pecten solaris_ fetched £4: 5s., _P. Reevii_ £4: 8s., and _Cardita varia_ 5 guineas. Mr. Dennison’s specimens of _Pecten subnodosus_ sold for £7, of _Corbula Sowerbyi_ for £10, of _Pholadomya candida_ for £8 and £13, while at the Vernède sale a _Chama damicornis_ fetched £7. CHAPTER V REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE FERTILISED OVUM--DIFFERENCES OF SEX--DIOECIOUS AND HERMAPHRODITE MOLLUSCA--DEVELOPMENT OF FRESH-WATER BIVALVES Reproduction in the Mollusca invariably takes place by means of eggs, which, after being developed in the ovary of the female, are fertilised by the spermatozoa of the male. As a rule, the eggs are ‘laid,’ and undergo their subsequent development apart from the parent. This rule, however, has its exceptions, both among univalve and bivalve Mollusca, a certain number of which hatch their young from the egg before expelling them. Such ovoviviparous genera are _Melania_, _Paludina_, _Balea_, and _Coeliaxis_ among land and fresh-water Mollusca, and _Cymba_ and many _Littorina_ amongst marine. The young of _Melania tuberculata_, in Algeria, have been noticed to return, as if for shelter, to the branchial cavity of the mother, some days after first quitting it. Isolated species among Pulmonata are known to be ovoviviparous, _e.g._ _Patula Cooperi_, _P. Hemphilli_, and _P. rupestris_, _Acanthinula harpa_, _Microphysa vortex_, _Pupa cylindracea_ and _muscorum_, _Clausilia ventricosa_, _Opeas dominicensis_, _Rhytida inaequalis_, etc. All fresh-water Pelecypoda yet examined, except _Dreissensia_, are ovoviviparous.