SECT, xv CLASSIFICATION OF CRUSTACEA 261 



As to the general truth of our theory that the 

 Ostracocla arc little more than folded Crustacean 

 heads with large head-shields, and with or without 

 a rudimentary dorsal shield, there can, we think, be 

 little doubt. We found strong confirmation of the 

 theory in the form of the closing muscles. It seemed 

 to us that if our view were correct, the closing muscles 

 must be modified from those which radiated from the 

 sternal plate in the transverse plane, and that they 

 ought, therefore, to show this origin. This surmise 

 was fully supported by the facts. The sinewy part 

 of the muscle is found in the centre the remains 

 of the sternal plate, from which the muscle fibres 

 radiate to the outer walls of the shells. If then 

 it is established that any of the Ostracoda are 

 descended from Trilobites, we have in this double- 

 headed closing muscle very clear proof that the 

 Trilobites possessed the sternal plate which we have 

 elsewhere assumed for them. 



As to the causes of the modification of some of 

 these primitive Trilobitic or Phyllopodan Crustacea 

 into Ostracoda, we may perhaps make the following 

 conjecture, borne out by the rudimentary condition 

 of the abdomen, and the small number of trunk 

 limbs. We have only to assume that in some of the 

 larvae of these primitive Crustaceans with head-shields, 

 the gradual thickening and stiffening of the chitinous 

 head-shield did not keep pace with the developing 

 muscles, whether the powerful mandibular muscles of 

 an early Apus, or the muscles of the masticatory 

 and springing first trunk limb of a Trilobitc. This 



