. III. DIST. CHAR. Prototype. 93 



external covering of the original; this may be 



70. Squamal (squamale) belonging to the 

 scales, as in petrified fish and amphibia 



71. Crustal (crustale) belonging to the crust ; 

 as in petrified insects-^ and echini;^ 



&c. &c. 



f In insects, the external crust or covering is considered, by ana- 

 tomist, as analogous to the bones, in the higher classes of animals, 

 ^f The covering of the Echinus may be divided into the 

 crust and its appendices. 



The CRUST (crust a) is the hard, calcareous, bone-like integu- 

 ment, in which the animal is enveloped. Its surface is generally 

 covered with tubercles, to which are articulated moveable spines of 

 various shapes and sizes. 



fl.)Mouth (os) the aperture of the crust through which the in- 

 closed animal takes its food. It is quiiique valve, and always 

 situate in the base of the shell. 



Vent (anus} the aperture through which the excrements of the 

 animal are passed. Its situation is various, being more or 

 less distant from the mouth. It is freqently closed by a kind 

 of operculum or lid ; and hence is not always visible in the 

 fossil subject. 

 #.)Base (basis') the bottom part of the crust, in which the aperture 



of the mouth is placed. 



Apex (apex} the top of the crust, opposite to the mouth. 

 Ambit (ambitus) the outline of the crust when placed on its 



base, and viewed from the top. 



Margin (margo) the edge formed by the crust's taking a com- 

 pressed or flattened shape. This is whole, dentate, sinu- 

 ate, &c. according to the species. 



4?.;Pores (pori) innumerable minute holes with which the shell is 

 regularly perforated. These are of two kinds. Through 

 the one, slender and almost imperceptible ligaments and fibres 

 pass, by which the spines are attached to the tubercles, and 



