636 CLASS x. 



Family IX. Balanoidea. Shell sessile, truncato-conical or 

 tubular, calcareous, open at the apex; calcareous valves at the 

 aperture. Animal conical, sometimes depressed. (Genus Balanus 

 BRUG.). 



Sea- Acorns, SearTulips. The pieces of shell, that form the calca- 

 reous tube, shut upon each other with teeth. In some the base is 

 closed by a calcareous plate, in* which hollow tubes run from the 

 middle to the margin, which communicate with other tubes in the 

 length of the shell. POLI Testae, utriusque /Sicilies, I. Tab. iv. figs. 

 9, 10. [Branching and inosculating ovarian tubes. DARWIN Bala- 

 nidce, pp. 100, 101.] 



[To obviate the extreme confusion of the nomenclature, DARWIN 

 proposes the following names for the external parts of Balanoids. 



What is visible externally of sessile cirripeds is composed of 

 shell and operculum, the operculum being generally seated a little 

 within the orifice of the shell. The shell consists of basis or support 

 by which it is attached (membranous or shelly), and of compart- 

 ments (8 4) occasionally all calcified together. The compartment 

 as the end of the shell where the cirri are exserted is called carina, 

 that opposite to it rostrum, those on the sides are the three lateral 

 compartments, that next the carina the carino-lateral, that next the 

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

 partment. These three are rarely present together. Each com- 

 partment consists of a wall (paries) which always grows downwards 

 and forms the basal margin, and is furnished on the two sides with 

 alee or with radii, or with an ala on one side and a radius on the 

 other. The radii, not always developed, on the upper part over- 

 lap the alee, which usually extend about half-way down the 

 compartment. The carina has always two alee. The carino- 

 lateral and lateral compartments have always an ala on the rostral 

 side and a radius on the carinal. The rostro-lateral (when present) 

 always radii on both sides. The rostrum normally has alee on 

 both sides, but very often, when fused with the rostro-lateral com- 

 partments, it has radii on both sides. The operculum consists of a 

 pair of scuta and a pair of terga, joined to the sheath of the shell by 

 the opercular membrane. On the internal surface of the scutum 

 there is almost always a pit for the attachment of the Adductor scu- 

 torum muscle, and beneath the adductor ridge, often a pit for the 

 depressor later alw muscle. 



