CEPHALOPODA. 71 



with simple parietes ; the volutions wholly visible, exposed, 

 and contiguous ; with transverse, numerous, and imperforate 

 septa. 



Discorbis vesicularis. — The Vesicular Discorbis. 

 Plate X. fig. 13. Volutions nodulous, subvesicular ; the 

 last volution sometimes closed. Fossil at Grignon. 



FAMILY III. RADIOLACEA. 



Shell discoid, with a central spire, and elongated radiating 

 cells, extending from the centre to the circumference. 



Genus 17— PLACENTULA._Z«marc^. 



Generic Character. — Shell orbicular, convex above and 

 below, multilocular ; aperture oblong, narrow, disposed like 

 a radius in the lower disk, or sometimes in both upper and 

 under. 



Placentula asterisans — The Star-shaped Placentula. 

 Plate X. fig, 14. Volutions convex ; partitions radiating 

 from the centre. Fossil, 



Genus 18 LENTICULITES.— iamarcA. 



Generic Character — Shell sublenticular, spiral, multi- 

 locular; outer margin of the volutions complicated, and 

 extending on both sides to the centre ; septa imperforate, 

 curved, and lengthened on both sides in the form of rays ; 

 aperture narrow, projecting over the last volution. 



Lenticulites rotulata The Rolled Lenticulites, 



Plate X, fig. 15. Orbicular; margin acute; disks some- 

 what prominent. Fossil at Mendon. 



Genus 19.— ROTULITES.— ZaTwarc^, 



Generic Character — Shell orbicular, multilocular, spiral, 



convex, or conical, above ; flat, radiated, and tubercular 



beneath; aperture marginal, trigonal, and directed upwards. 



Rotulites trochidiformis. — The Trochus-shaped Ro- 



