CORONULA. 37 
valve. The basal margin forms an angle rather above a right angle with the spur. _Inter- 
nally the articular ridge and crests for the depressor muscles, feebly developed. 
Internal Structure of the Shell and Basis.—Internally, the shell is ribbed, more or less 
prominently. The lower edge of the sheath, which is reddish, and extends far down the 
walls, seems always to project freely. In several specimens there were on each side, at 
the carinal end of the shell, a trace apparently of a suture, which could be perceived only 
on the sheath. The basis appears always to be permeated by minute tubes or pores, 
though these are sometimes rather difficult to be seen. 
Michelotti, in the ‘Bulletin Soc. Géolog.’ Tom. 10, p. 141, has named, but not 
described, a species, viz., Pyrgoma undata, from the northern Italian Tertiary Strata. 
Genus—CoRONULA. 
Coronuta. Lamarck. Annales du Muséum, tom. i, (1802). 
Diappema. Schumacher. Essai d’un Nouveau Syst., &c., 1817. 
CETOPIRUS (SED NON CoronuLa). Ranzani. Memoire di Storia Naturale, (1820). 
Potytepas. J. H. Gray, (Klein). Annals of Philosophy, (new series), vol. 10, 1825. 
Valve taste 6, equali latitudine; parietes tenues, profundé plicati, plicis cavitates 
infra solim apertas eficientibus ; valve operculares non inter se articulate, orificio teste 
multo minores: basis membranacea. Cetaceis affiva. 
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; when both present not articulated together: basis membranous. 
Attached to Cetaceans. 
The structure of the shell of Coronula is complicated, and has been generally quite mis- 
understood. Without a long description and several figures it would be impossible to 
give a true idea of its singular structure ; but, in order to make the following description 
at all intelligible, I must make a few remarks. The wall of each compartment, and there- 
fore of the whole shell, is extremely thin; but strength is gained by its being folded in a 
very complicated manner, as may be seen in the rostral compartment, ‘Tab. II, fig. 84, 
by tracing the walle to ¢, to e”; the folds at their outer ends are elongated into transverse 
loops, the extremities of which touch each other; consequently, what appears to be the out- 
side of the shell consists only of a portion of the wall, namely, the outsides of the transverse 
circumferential loops, together with the radu. These loops appear externally like much 
flattened longitudinal broad ribs. On the other hand, the inside of the shell, in which the 
body is lodged, consists of the inner ends of the folded walls, lined by the sheath, and by 
the ale. The basal edges of the folded walls, in the line of the ray of the circular shell, 
are oblique; the outer ends, or transverse circumferential loops, having grown downwards 
at a greater rate than the inner ends. Between each fold of the walls, there is a flattened 
