GENUS CORONULA. 397 



the level of the parietes, the outer lamina of the radius has a considerable 

 thickness, and is sometimes separated from the inner portion of the 

 radius by an interval, in the same manner as in C. testudinaria ; 

 and the growing edges of the outer lamina exhibit traces of the same 

 mamillated structure as figured in that species. 



The Opercular Valves are apt to be rather narrower and more elon- 

 gated than in the foregoing species, with the occludent margin of the 

 scutum generally, but not always, only slightly sinuous : externally the 

 scutum is sometimes feebly striated longitudinally. In the tergum 

 (PI. 14, fig. 3 h) the longitudinal furrow is generally plainer, from 

 being more distant from the carinal margin, and the rudimentary spur 

 itself is more prominent. 



The mandibles usually have five narrow teeth ; but I have seen one 

 specimen having only four teeth, and with the inferior angle truncated. 

 The cirri present no particular character. 



SECOND SECTION OF THE SUB-FAMILY OF BALANINiE. 



\_Scutum and tergum (when both are present) not overlap- 

 ping each other, or articulated together ; basis membranous ; 

 parietes often deeply folded, with the outer lamina, towards 

 the basis, generally imperfect ; each branchia composed of 

 two plicated folds ; shell attached to living vert ebr at a?\ 



8. Genus — Coronula. PI. 15, 16. 



Coronula. Lamarck. Aunales du Museum, torn. ] (1802). 

 Diadema. Schumacher. Essai d'uu Nouveau Syst., &c, 1817. 

 Cetopirus (sed non Coronula). Ranzani. Memoire di Storia 



Naturale (1820). 

 Polylepas. J. E. Gray, (Kfein). Annals of Philosophy, (new 



series), vol. 10, 1825. 



Compartments six, of equal sizes: walls thin, deeply 



folded, with the folds forming cavities, open only on the 



under side of the shell : opercular valves much smaller than 

 the orifice of the shell. 



Distribution, mundane, imbedded in Cetacea. 



