384 
MR. W. Iv. PARKER AND PROFESSOR T. R. JONES ON SOME 
Planorbulina vulgaris grows on rough shells (such as Tridacna and IlipflOflus) ; and its 
under surface touches but at points, not lying flat (as in PI. Me diterranensis on sea-weed- 
fronds, and PI. lobatula on smooth shells and algae). 
In the fossil state we have PI. coronata from the Grignon sands (Eocene), of large 
size, rivalling in size the Discorbina trochidiformis of that deposit. 
Under the names of Botalia , Bosalina , Anomalinci, and Truncatulina , have been 
described a great number of subnautiloid forms which are evidently some of them enfee- 
blements of PI. coronata (the nearest being Truncatulina vermiculatci , D’Orb. Foram. 
Amer. Merid. pi. 6. figs. 1-3), whilst others are either young or arrested modifications 
of PI. vulgaris. In deep water Planorbulina scarcely ever takes its true Planorbuline 
character ; this many-chambered condition seeming to require sea-weeds or shell-surfaces 
for support. Mixed with these, and at still greater depths, we get numbers of small 
subsymmetrical nautiloid forms of this species, such as have passed under the names of 
Botalia Clementiana , D’Orb., and Botalia ammonoides, Reuss; as well as many other 
forms ranging between the latter and Planulina Ariminensis. Planorbulina ammonoides 
of the Lias, Gault, and Chalk takes on the symmetrical (subnautiloid) character so 
distinctly as to be mistaken for small Nonionince. These small, more or less symmetrical 
Planorbulince, so common in some deposits of the Secondary period, are abundant enough 
in the existing seas at from 100 to 1000 fathoms, or even more. We may suppose that 
the sea-weeds and bivalves of the shallow water of the Secondary period were abundantly 
encrusted with Planorbulince as littoral representatives of the deep-sea forms now fossil 
in the clays of that period. 
Genus Discorbina. 
Discorbina Turbo , D’Orbigny (Varieties). Plate XIV. figs. 18-23 (Arctic) ; Plate XVI. 
figs. 26-28 (North Atlantic). 
Discorbina presents a simple Rotaline form of shell, having from 7 to 30 more or less 
vesicular chambers, with double septa when the chambers are discrete, and with rudi- 
ments of the canal-system. The shell is coarsely porous (coarser than that of Cymbalo - 
pora, and less so than Planorbulina) ; somewhat conical in shape ; the upper side the 
thickest ; the margin rather sharp ; but some varieties are complanate with square edges. 
The aperture is a large arched slit, usually occluded by an umbilical process or flap, 
which is sometimes developed into a subsidiary umbilical chamber ; and the flaps or 
chamberlets of the successive chambers give a star-like or Asterigerine aspect in the 
umbilicus. Exogenous shell-growth sometimes thickens the septal lines of the spire ; 
but it frequently ornaments, and even masks, the umbilical lobes. 
The many varieties of this porous and flap-bearing Rotaline species are so intimately 
connected one with the other, that the following classification is little more than 
suggestive and provisional. 
