2i 



CHAPTER III. 



OF THE TERMS USED IN DESCRIBING SHELLS. 



It is to be observed in explaining the terms made use of in 

 describing shells, that the same word has sometimes a very 

 different meaning from what it is intended to convey at others; 

 but if the connection in which it is used is attended to, there 

 will seldom occur much difficulty in knowing how to under- 

 stand it. For instance, the term ovata occurs in the twelfth 

 edition of Systema Naturae, and in the eighth volume of the 

 Transactions of the Linnaean Society, in describing the form of 

 Mija truncata and arenaria, and also of Bulla fontinalis and 

 Hypnormn, although the first are of a very different shape from 

 the last. Also Cyprcea Onyx, C. Ziczac, &c. are said to be um- 

 hilicata, as well as Nerita Caurena, glaucina, &c., though the 

 hollow which is called the uinbilicus is so exceedingly different 

 in form and structure in the first from what it is in the last. 



OF BIVALVE SHELLS. 



First, of the Substance, which is either — 

 Crassiis, thick, as Vemis verrucosa. 

 Tenuis, thin, as Tellina depressa and Fabula. 

 Memhranacea, thin like a skin, as Solen anatinus. 

 Fragilis, brittle, as Solen JLegumen. 

 Pellucidus, transparent, as Tellina lactea and lacustris. 

 Opacus, opake, as Cardium edule. 

 Margaritaceus, pearly, as Mytilus margaritiferus. 



