TYPE WITH EARLY DEVELOPMENT OF THE ADULT PYGIDIUM. 339 



BARRANDE'S fourth type. The cephalic region and the thorax are 

 here already completely developed ; the pygidium, on the contrary, 

 is somewhat undeveloped, the number of segments composing it 

 being still incomplete. We must class under this type Paradoxides, 

 the species of Dalmanites which belong to the Hausmann group, a 

 few species of Phacops, Proetus, Ataphus, etc. 



2. Type with early development of the adult pygidium. 



This division corresponds to the second of BARRANDE'S ontogenetic 

 types, to which belong Agnostus and Trinucleus. The youngest 

 known stages consist merely of the rudiments of the cephalic shield 

 and the pygidium. The latter, although still incomplete, has essen- 

 tially the characters of the adult. Metamorphosis is thus limited 

 to the development of the thoracic region, which takes place in 

 such a way that, as in forms already described, successive free 

 segments become cut off from the anterior part of the pygidium. 

 The other modifications consist in the increase in the number of 

 the rudiments of the segments in the pygidium, and in the more 

 perfect shaping of the cephalic region. In Trinucleus, for example, 

 the characteristic rows of pores of the "limb" of the head-shield 

 are developed. 



This form of development, as contrasted with the earlier type, must be 

 regarded as more specialised. In view of the evident importance of the 

 presence of the pygidium, 

 we cannot greatly wonder 

 that the modification of 

 the posterior segments 

 of the body to form this 

 structure was shifted to 

 quite an early stage. In 

 the small number of 

 segment - rudiments 

 in the first stages, the 

 abstriction of the tho- 

 racic segments and the 

 development of new rudi- 

 ments of segments at 

 its anterior end, the 

 pygidium, in the younger 

 stages of this type, closely 

 resembles the transitory 

 pygidium of the types 



described above. It is, however, distinguished from these by the fact that, in 

 form, it more nearly approaches that of the adult. It is evident that no sharp 

 distinction can be made between these two types of development. 



The ontogenetic stages of Agnosias and of Trinucleus strikingly recall certain 



FIG. 153 Four stages in the development of A[iH".<fi(s nudus 

 (after BARRANDE). A, youngest stage, consisting of the 

 cephalic shield and the pygidium. B, sta^e with the rudi- 

 ments of two thoracic segments. C, stage with two fully 

 formed thoracic segments. D, adult form. 



