CEPHALOPODA, 61 



Discorhis vesicularis. — The Vesicular Discorbis. Plate 

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

 lution hidden by the nodules. 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.— ZamarcJ^. 



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.— LENTlCUhm A.— Lamarck. 



Generic Character. — 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 ; disc globular. Fossil 

 at Mendon. 



Genus 19.-R0TULITES.— iowarc/^. 



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

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

 aperture marginal and trigonal. 



RotuUtes trochidiformis. — The Trochus- shaped Rotulites. 

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

 one granulated. Fossil at Grignon, 



FAMILY IV. SPHERULACEA. 



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

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



Genus 20.— MEL ONI A.— Zawarc*. 



Generic Character — Shell subspherical, multilocular; spire 



