336 



PALAEOSTRACA. 



the posterior growing mass, the most anterior of these rudiments 

 becoming segmented off as the free, movable thoracic somites. 

 Fresh thoracic segments are successively set free from the anterior 

 end of the posterior mass, which itself continues to grow and form 

 new segment-rudiments. This posterior mass of incompletely sepa- 

 rated segment-rudiments is nothing more than the budding zone of 

 the still undeveloped thoracic segments, and must not be confounded 

 with the pygidium of the adult. It was called by BARRANDB the 

 "pygidium transitoire," and is distinguished externally from the 

 adult pygidium by the fact that the latter, in Sao and Dalmanites, 

 has an unbroken margin, while the posterior margin of the transitory 

 pygidium shows freely-projecting teeth which are connected with the 



C 



FIG. 151. Five ontogenetic stages of Olenellus asaphoides (after FORD). A and B, younger 

 stages. C and D, older stages. E, adult form, a, cheek-spine ; b, inner spine ; c, optic 

 rudiment ; d, swelling within that rudiment ; p, transitory pygidium. 



development of the pleura of the free thoracic segments. Only 

 after the full number of free thoracic segments (in Sao 17) is 

 attained does the adult pygidium develop; this, in Sao, is very short. 

 While the full segmentation of the adult is in this way gradually 

 attained, the cephalic region, apparently through modifications 

 brought about simply by growth, undergoes transformation which 

 causes it to approach nearer to the shape of the adult head-shield. 

 The limbs, the occipital furrow, and the cheek (genal) spines appear. 

 The glabella becomes more distinct, and the transverse furrows appear 

 on it, giving a suggestion of segmentation. Finally, the facial suture 

 (g) can be distinctly recognised, and the granulated ornamentation of 



