216 PALEONTOLOGY 



CLASS: INSECTA (Hexapoda). 



With head bearing antennae, mandibles and two pairs 

 of maxillae, thorax with 3 pairs of appendages and 2 pairs 

 of wings, and abdomen without appendages. 



Notes on the Chief Classes and Orders of 

 Arthropoda (other than Trilobites) found Fossil. 



The Phyllopoda are mostly non-marine, but as common 

 in excessively salt as in fresh waters. The form of the 

 body and the appendages have some resemblance to those 

 of a primitive trilobite like Olenus ; but (i) there is no 

 trilobation ; (2) no fusion of posterior somites into a 

 pygidium ; and (3) the exo- skeleton of thorax and abdomen 

 is thin, and to protect the hinder regions the head-shield 

 is extended back as a loose covering. In some forms, 

 especially in the fossil Estheria, this head-shield is divided 

 into two by a median suture so as to form practically a 

 bivalve shell : this being ornamented by sub-concentric 

 ridges closely resembling the growth-lines of a mollusc, 

 it is very easy to mistake Estheria for such a lamelli- 

 branch as Posidonomya. All the more is this the case, 

 because while, in general, arthropod shells do not grow 

 marginally but are only replaced by ecdysis, Estheria is 

 stated to be a unique exception, in which the concentric 

 lines are actually growth-lines. The following points 

 should be looked for to distinguish Estheria : (i) There is 

 no hinge-structure as in lamellibranchs ; (2) the material 

 of the shell is not calcareous but chitinous; (3) with 

 a lens there may be seen between the apparent growth- 

 lines a network of striae which is never seen in lamelli- 

 branchs. Estheria is common from Carboniferous to 

 Triassic. Protocaris is a Cambrian marine form scarcely 

 distinct from the modern freshwater Apus. 



