54 INTRODUCTION. 



14. Convolutce (plates, fig. 444 to 462). The well-known 

 shells contained in this family are distinguished for the 

 small proportion of the spire, if any, which remains un- 

 covered by the last whorl. They might be well divided 

 into two groups, the first containing the genera Ovulum 

 and Cypraea, under the name of Cypraeadse, which are 

 truly convolute, having the spire entirely hidden ; and 

 the second containing the genera Oliva, Ancillaria, and 

 Conus. 



Order Polythalamous, or Chambered Cephalopoda. ' 



The greater part of the shells belonging to this order are 

 symmetrical, and the internal cavity is divided into separate 

 compartments, by plates called Septa. It is divided into the 

 following families : — 



1. Orthocerata (plates, fig. 463 to 470), containing the 



genera Belemnites, Orthoceras, Nodosaria, Hippurites, 

 and Conilites. Hippurites certainly has no affinity with 

 the Cephalopoda, but is ascertained to be a bivalve 

 shell, properly belonging to the family Rudistes; the 

 other genera are straight, elongated, and conical. 



2. Lituacea (plates, fig. 471), containing the genera Spirula, 



Spirulina, and Lituola, the two latter of which are 

 microscopic. 



3. Cristacea, containing the microscopic genera Renulina, 



Orbiculina, and Cristellaria. 



4. Spherulacea, containing the microscopic genera Miliola, 



Gyrogona, and Melonia. 



5. Radiolacea, containing the microscopic genera Rotalites, 



Lenticulina, Placentula. 



6. Nautilacea (plates, fig. 472 to 476). This family con- 



tains the following genera — Discorbites, Siderolites, 

 Polystomella, Vorticialis, Nummulites, and Nautilus; 



