4 INTRODUCTION. 



operculum or opercular valves [valvce opercular es) , and the 

 shell {testa), though these parts homologically present no 

 real difference. The operculum is seated generally some 

 little way down within the orifice of the shell • but in very 

 young specimens and in Verruca, the operculum is attached 

 to the summit of the shell, and the shell, without the 

 operculum be removed, can hardly be said to have any 

 orifice ; though, of course, the opercular valves themselves 

 have an aperture for the protrusion of the cirri. 



The shell consists of the basis (called the support by 

 some authors), which is membranous or shelly, and flat or 

 cup-formed, and of the compartments [testa valvce), which 

 vary from eight to four in number, and occasionally are all 

 calcified together. 



The compartment, at that end of the shell where the cirri 

 are exserted through the aperture or lips of the operculum, 

 is called the carina (fig. 1) ; the compartment opposite 

 to it is the rostrum, — these two lying at the ends of the 

 longitudinal axis of the shell. Those on the sides are the 

 lateral compartments ; that nearest the carina, being the 

 carino-lateral (testa? valva carino-lateralis), that nearest the 

 rostrum, the rostro -lateral, and the middle one, simply the 

 lateral compartment ; but these three compartments are 

 rarely present together. The rostro-lateral compartment, 

 which always resembles fig. 2, and may be always known 

 by having radii on both sides, is often absent; and not 

 rarely the lateral and carino-lateral compartments are con- 

 founded together, or one is absent ; in such cases the com- 

 partment that is left is simply called the lateral one. The 

 compartments are separated from each other by sutures, 

 which are often so fine and close as to be distinguished with 

 difficulty. The edge of a compartment, which can only 

 be seen when disarticulated from its neighbour, I have called 

 the sutural edge (acies suturalis). 



Each separate compartment consists of a wall (paries), or 

 parietal portion (pp in woodcuts), which always grows down- 

 wards, and forms the basal margin ; and is furnished on the 

 two sides either with ala (fig. 4), or with radii (fig. 2), 

 or with an ala on one side and a radius (fig. 3) on the 

 other. 



