SECT. XIII 



THE TRILOBITES 



213 



they lie outside the longitudinal line which joins the 

 other limbs. The development of hard cuticular 

 points, and thence of thorns on such lateral projec- 

 tions, would be but a matter of time. From these 

 points also the gradual development of the ridge 

 round the front of the head can easily be imagined. 

 In some Trilobites it remains quite inconspicuous, but 



FIG. 47. Dionide formosa (Barr), showing the giabella and the gradual rudimentary 

 character of the posterior segments. 



in others, as already stated, it projects as a great 

 shovel-shaped margin. We here find, then, the origin 

 of all forms of the Crustacean shell, which we have 

 deferred discussing till now ; we may summarise our 

 conclusions as follows : 



Round these lateral projections, due to the bending 

 of the cylindrical bod}-, all the shapes of the Trilobite 



