GENUS CORONULA. 407 



formed of concentric slips : each slip has eighteen angles, 

 corresponding with the open ends (see PL 16, fig. 5) of 

 the eighteen folds of wall. A ray of membrane rims under 

 each of these folds, being prolonged from the basal mem- 

 brane ; but these rays can hardly properly be called parts 

 of the basal membrane, for they split at each period of 

 growth along the middle, and the two halves are drawn from 

 under the basal edges of the walls, and thus come to invest 

 their outer surfaces. The basal membrane, and the whole 

 cementing apparatus, which is much simpler than in Balanus, 

 has been fully described (p. 135) in the Introduction. 



Opercular Valves. — In the sessile cirripedes hitherto ex- 

 amined, the four opercular valves 'are inclined towards each 

 other, and nearly fill up the orifice of the shell, being united 

 to the walls by a more or less narrow circular border of mem- 

 brane ; this membrane being attached rather low down to the 

 sheath. In Coronula the opercular membrane is stretched like 

 the skin of a drum, almost horizontally across the top of the 

 shell ; it is, however, generally attached to the sheath rather 

 lower down at the carinal than at the rostral end : hence the 

 animal's body, as remarked by Burmeister, is attached al- 

 most horizontally; but this we shall see, under Xenobalanus 

 and Tnbicinella, cannot be considered a character of much 

 importance. The valves are quite small compared with the 

 opercular membrane, and certainly are of little functional 

 importance. The scuta in C. balaenaris and diadema (PL 1 5, 

 fig. 3 b) are sub-triangular ; but the under or growing sur- 

 face is elongated and arched. These valves stand almost 

 at the rostral end of the orifice, instead of on each side of it. 

 In C. balcenaris the terga are small, with the under surface 

 oval: in C. diadema they are either quite aborted, or are repre- 

 sented by a barely visible plate of shell, parallel and close to 

 the tergal margin of the scutum. The aperture leading into 

 the sack is formed by a nearly medial slit of considerable 

 length, furnished with irregularly protuberant, inwardly in- 

 clined lips. These lips are formed by the development of 

 an inner fold or crest of membrane, which can be just 

 detected in most Cirripedes : the lips include a double fold 

 of corium, and are covered by a delicate tunic, continuous 

 with that lining the sack, and homologically continuous 



