134 



THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



ORIGIN OF THE PYGIDIUM. 



Taking first the pygidium, it has already been pointed out that in each case the pygidium 

 of the adult is proportionally considerably smaller than the pygidium of the protaspis. The 

 stages in the growth of the pygidium are better known in Sao hirsuta than in any other 

 trilobite, and a review of Barrande's description will be advantageous. 



Barrande recognized twenty stages in the development of this species, but there was 

 evidently a still simpler protaspis in his hands than the smallest he figured, for he says, 

 after describing the specimen in the first stage : "We possess one specimen on which the 

 head extends from one border to the other of the disk, but as this individual is unique 

 we have not thought it sufficient to establish a separate stage." This specimen is important 

 as indicating a stage in which there was not even a suggestion of division between cephalon 

 and pygidium. 



In the first stage described by Barrande, the form is circular, the length is about 0.66 mm., 

 and the glabella is narrow with parallel sides and no indications of lateral furrows. The 

 neck segment is indicated by a slight prominence on the axial lobe, and back of it a con- 

 striction divides the axial lobe of the pygidium into two nodes, but does not cross the 

 pleural lobes. The position of the nuchal segment permits a measurement of the part which 

 is to form the pygidium, and shows that that shield made up 30 per cent of the entire 

 length. 



In the second stage, when the test is 0.75 mm. long, the cephalon and pygidium become 

 distinctly separated, and the latter shield shows 'three annulations on the axial and two 

 pairs of ribs on the pleural lobes. It now occupies 33^ per cent of the total length. 



In the third stage, when the total length is about i mm., the pygidium has continued 

 to grow. It now shows five annulations on the axial lobe, and is 46 per cent of the total 

 length. 



In the fourth stage, two segments of the axial lobe have been set free from the front 

 of the pygidium. The length is now 1.5 mm. and the pygidium makes up 32 per cent of the 

 whole. From this time the pygidium continues to decrease in size in proportion to the total 

 length, as shown in the following table. 



