Page 243 - Introduction to Paleobiology and The Fossil Record

230 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.11 Classifi cation of siliceous-walled groups: diatom classifi cation

Two main divisions are recognized based on their shell morphologies: the Centrales, as the name
suggests, have round valves with pores radiating in concentric rows from the valve center; the Pen-
nales have more elliptical valves with the pores arranged in pairs (Fig. 9.20). The latter are usually
characterized by a median gash or raphe.

Order CENTRALES
Suborder COSCINODISCINEAE
• Valves with a ring of marginal processes
Suborder RHIZOSOLENIINEAE
• Valves are unipolar
Suborder BIDDULPHIINEAE
• Valves are bipolar

Order PENNALES

Suborder ARAPHIDINEAE
• Valves without a raphe
Suborder RAPHIDINEAE
• Valves with a raphe

CENTRALES

Coscinoconus Asterolampra

PENNALES

Cocconeis

Surirella
Achnanthes
Eunotia

Figure 9.20 Some diatom morphotypes: Coscinoconus (×250), Asterolampra (×400), Cocconeis
(×360), Achnanthes (×150), Surirella (×200) and Eunotia (×400).