382 
MOLLUSCA. 
gined with unequal radiating lobes ; sej)ta transverse, imperfo¬ 
rate : aperture distinct, somewhat lateral. 
Siderolites calcitrapoides _ The Chalk Siderolites. 
Plate LXXXIII. fig. 12. 
With four lobes, and punctated throughout. Fossil in the 
mountain of St Peter, at Maastricht. 
Genus 16.—DISC ORB IS.— Lamarck. 
Generic Character —Shell discoid, spiral, multilocular, with 
simple partitions; the volutions wholly visible, exposed, and 
contiguous ; with transverse, numerous, and imperforate septa. 
Discorbis visicularis — The Visicular Discorbis. 
Plate LXXXIII. fig. 13. 
Volutions nodulous, subvisicular ; the internal volution hid¬ 
den by the nodules. Fossil at Grignon. 
TRIBE III.-RADIOLACEA. 
Shell discoid, with a central spire, and elongated radiating 
cells, extending from the centre to the circumference. 
G enus 17.—PLACENTULA. — Lamarck. 
Generic Character. —Shell orbicular, convex above and below, 
multilocular; aperture oblong, narrow, disposed like a radius in 
the lower disc, or sometimes in both upper and under. 
Placentula astricans. — The Star- Shafed Placentula. 
Plate LXXXIII. fig. 14. 
Volutions convex; partitions radiating from the centre. 
Fossil. 
Genus 18—LENTICULIN A.— Lamarck. 
Generic Character .—Shell sublenticular, spiral, multilocular; 
outer margins of the volutions complicated, and extending above 
and below to the centre; septa imperforate, curved, and length¬ 
ened on both sides in the form of rays; aperture narrow, pro¬ 
jecting over the last volution. 
