EEPORT ON THE EADIOLAETA. 1741 



the Ccelographida essentially the same form and structure as in the preceding Coeloden- 

 drida. The only (but important) difference between them is indicated by the constant 

 presence of the peculiar rhinocanna in the former, whilst this is always absent in the 

 latter. The two valves of the shell, dorsal and ventral, are either hemispherical or some- 

 what flatter, sometimes nearly cap-shaped, and formed of an extremely delicate and irregu- 

 larly fenestrated plate of silica, as in the Ccelodendrida. As in the latter, so also in the 

 Coelographida both valves are of similar form and usually of equal size, but sometimes 

 the dorsal is a little smaller than the ventral valve. The remarkable difference which 

 Biitschli describes in his Ccelothamnus davidoffii, and the inverse origin of the three tubes 

 in both valves (loc. cit., Taf. xxxi. figs. 2, 4), depends upon an error of observation, 

 produced by the artificial inversion of one valve, and the dislocation of their natural 

 arrangement. The valves are never in direct contact, but separated by the equatorial 

 fissure or girdle-cleft, in which the girdle zone of the enclosed central capsule and its 

 three openings lie freely (PI. 127, figs. 4, 5; PI. 128, fig. 2). The free margins of 

 both valves, which are opposite to one another, and bound the girdle-cleft, are always 

 equidistant, so that the cleft in the whole equatorial circumference is of equal breadth. 

 The margins are usually irregularly denticulate, sometimes armed with longer bristles 

 (PI. 127, fig. 8), more rarely smooth (fig. 5). The delicate lattice-work of the valves 

 is always irregular and very variable, usually with numerous small and unequal pores, 

 sometimes rudimentary, so that the valves appear partly solid and hyaline. The size 

 of the valves is usually between 0'2 and 0'5 (in diameter). 



The galea (g) or the apical cupola, which arises from the vaulted apex of each valve (or 

 its sagittal pole) is more developed in the Coelographida than in the preceding Coeloden- 

 drida, and differs from the latter in the peculiar rhinocanna arising from its base, and in 

 the single or double frenulum, connecting the open mouth of the rhinocanna with the 

 odd or paired main tube arising from the galea. The two opposite galese lie therefore 

 on the poles of the sagittal axis of the bivalve shell, and are so symmetrically disposed 

 in the sagittal plane, that the open mouths of their rhinocannse are directed towards the 

 oral pole of the main axis, and nearly come in contact with the proboscis arising from 

 the radiate operculum of the central capsule (PI. 127, figs. 4, 5). 



The size and form of the galea are very variable, even in one and the same species. 

 The volume of its cavity is generally about as great as that of the hemispherical valve 

 from which it arises, sometimes larger, at other times smaller. Its fundamental form is 

 constantly dipleuric or bilateral, since the radial hollow tubes arise symmetrically on both 

 its sides, and the rhinocanna proceeding from its base determines the sagittal plane. 

 Usually the galea has the form of a vaulted helmet, the convex crest of which is inclined 

 towards the mouth (PI. 127, figs. 4, 5, 8, 9). Its anterior or apical part is broad and 

 truncated in the Coslotholida, more or less conical in the Coelospathida ; sometimes it 

 assumes .nearly the form of a bilateral three-sided pyramid, at other times it is more 



