THE NATURAL HISTORY SURVEY. 



177 



The principal changes which take place in the individual 

 during growth are: i. The gradual elongation of the body 

 by the successive introduction of free thoracic segments: 2. 

 The change in position of the eyes: 3. The modification of the 

 glabella. 4. The growth of the free cheeks. 5. The final 

 assumption of the mature specific characters of the pygidium, 

 and the ornamentation. 



FfG 5. 



Ptychopariakingi, Meek 

 Cambrian. A, Protaspis 

 enlarged. B, Adult re- 

 duced. (After Beecher). 



FIG 6. 



Sao Mrsuta, Barran 'e. 

 Cambrian. A, Protaspis 



FIG'. 7. 

 Triarthrus becki. Green. 

 Ordovician. A, Protaspis 



enlarged. B Adult re- enlarged. B, ' Adult re- 

 duced (After Beecher). duced. (After Beecjher) . 



B 



FIG 8. FIG. 9. PIG. 10. 

 Proetus fiarviusculus. Acidaspts tuberculata. Dalmanites socialis. Bar- 

 Hall. Odrovician. A, Pro- Conrad. lyOwer Devonian, nande. Ordovician. A, 

 taspis enlarged. B, Adult A. ProUspis enlarged. B. Protaspis enlarged. B. 

 reduced. (After Beecher). Adult red uce d . (After Adult reduced. (After 

 Beecher). Beecher). 



The simplest expression of the protaspis stage is found in 

 the earliest genera known, those of the Cambrian; in later 

 genera the process of acceleration or earlier inheritance lias 

 pushed forward the initiation of certain characters in the growth 

 of the individual until they appear in the protaspis, thus mak- 

 ing it far more complex than in the more primitive forms of 

 the Cambrian. Many of the earlier and more primitive genera 

 of trilobites, even in their adult form, retain certain characters 

 suggestive of the protaspis. Any genus of trilobites with the 

 cephalon large and elongate as compared with the pygidium, 

 with the eyes rudimentary or absent, with the free cheeks 

 ventral or marginal, with the glabella elongate, cylindrical, 

 and with five annulations, would naturally be considered as 

 belonging to a primitive stock. 



During the protaspis stage the individual passes through 

 several moults previous to the complete separation of the 

 pygidium or the introduction of thoracic segments, which 

 produce the stronger annulation of the axis, the appearance 

 of the free cheeks upon the dorsal side and the 1 increased seg- 



