CEPHALOPODA. 61 



DiscorUs vesicular is. — Tn^ Vesicular Discorbis. Plate 

 X. fig. 13. Volutions nodulous, subvesicular ; the internal vo- 

 lution hidden by the nodules. Fossil at Grignon. 



FAMILY III. RADIOLACEx\. 



Shell discoid, with a central spire, and elongated radiating 

 cells, extending from the centre to the circumference. 



Genus 17.— PLA CE NT UL A. —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-shaped Placentula. 

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

 the centre. Fossil. 



Genus 18.— LE NTI CU LIN A —ZawarcA. 



Generic CAamcfer.— Shell sublenticular, spiral, multilocular; 

 outer margins of the volutions complicated, and extending above 

 and below the centre ; septa imperforate, curved, and length- 

 ened on both sides in the form of rays ; aperture narrow, pro- 

 jecting over the last volution. 



Lenticulites rotulata.— The Rolled Lenticulites. Plate 

 X. fig. 15. Orbicular ; margin acute j disc globular. Fossil 

 at Mendon. 



Genus 19.— ROT ULITES.— /: a//mrcZ-. 



Generic Character. — Shell orbicular, multilocular, spiral, con- 

 vex, or conical, above ; fiat, radiated, and tubercular beneath ; 

 aperture marginal and trigonal. 



Rotulites trochidiformis. — The Trochus- shaped Rotulites. 

 Plate X. fig. 16. Shell conoid ; volutions carinated ; lowei 

 one granulated. Fossil at Grignon. 



FAMILY IV. SPHERULA.CEA. 



Shell globular, spheroidal, or oval ; with the volutions mutu- 

 ally enveloping, or the cells contiguous, and forming a tunic. 



Genus 20.— MELOl^l A.— LamarcJt. 

 Generic Character — Shell subspherical, multilocular; spire 



