294 AN INTRODUCTION TO THE STUDY OF FOSSILS 



ments, except the anal, bear a pair of appendages ; (3) all append- 

 ages, except the antennules, are biramous and are produced 

 into gnathobases which on the head are used as organs of masti- 

 cation ; (4) the digestive tube is a straight canal, apparently 

 without an anterior expansion into a stomach ; (5) typically, the 

 head gives indication dorsally of the five segments of which it is 

 composed ; (6) the earliest larval form (protaspis) of a trilobite 

 from the Cambrian is more primitive than the simplest larval 

 form (nauplius) of existing Crustacea. 



Derivation of name. — Trilobita < Latin tri, three, -globus, 

 a lobe, referring to the division of the dorsal shield into three 

 longitudinal portions, — the axial lobe and two pleural lobes. 



Paradoxides (Fig. 127). Middle Cambrian. 



The portion of the animal found fossil is the strongly calcified 

 dorsal shield, and very frequently also the upper lip (hypostome). 

 The glabella of the dorsal shield enlarges anteriorly with its lobes 

 well defined ; the genal angles are usually produced into spines 

 as are consequently likewise the tips of the pleurae ; the genal 

 spines are borne by the free cheeks ; the eye lobes are large and 

 arched ; the thorax has 17 to 20 segments ; the pygidium is 

 small, plate-like and segmented. The name was given by Bron- 

 gniart in 1822 out of compliment to Linne's name for a trilobite, 

 — Entomolithus paradoxus, the paradoxical insect. 



It is very abundant in North America and Europe. 



1. Sketch specimen, labeling cephalon, glabella, eye lobes, 

 genal spines, fixed and free cheeks, facial suture, thorax, axis, 

 segments of thorax, pygidium. 



2. Make an ideal sketch showing the relationship of the hypo- 

 stome to the rest of the animal. 



3. Why is this genus placed under the trilobites ? 



4. Was the skeleton external or internal ? How much of 

 the entire animal is preserved in the specimen sketched ? 



5. What was the composition of the skeleton in the living 

 state ? What is its composition now ? 



6. Does this specimen necessarily represent the death of an 

 animal ? Why ? 



