338 



PALAEOSTRACA. 



explained. When we consider the ontogenetic stages of Olencllus (Fig. 151 C 

 and D), we see that the pleurae of the free thoracic segments do not at first 

 extend laterally beyond the middle of the cephalic region, as above described. 

 We are therefore led to ask whether the inner spine (&), alluded to above, is 

 not to be referred to the category of pleurae of one of those posterior segments 

 (i.e., trunk-segments) which have fused secondarily with the glabella. 



The later ontogenetic stages of Olenellus asaplwides are marked by the fact 

 that the pleurae of the third free thoracic segment appear greatly lengthened 

 posteriorly (Fig. 151 C' and D), a feature wanting in the adult, but recurring in 

 the anterior segments of a few species of Paradoxides. Since the young stages 

 of Olenellus further agree with these species of Paradoxides in the notching 

 of the posterior margin of the cephalic shield, the metamorphosis of the former 

 seems to suggest certain stages in the phylogeny of the latter genus. 



The third ontogenetic type of 

 BARRANDE contains those forms 

 whose stages of development ex- 

 hibit the same characters as the 

 later stages of Sao hirsuta. The 

 cephalic region usually already 

 shows the adult form ; the number 

 of the thoracic segments is, how- 

 ever, incomplete, while the posterior 

 region of the body, functioning as 

 transitory pygidium, has still to 

 yield, by segmentation of its anterior 

 portion, the free thoracic segments, 

 which are at present wanting. 

 Further development takes place 

 as in the first type. We must 

 assume that BARRANDE'S third type 

 contains forms whose youngest 

 stages are not known, or those 

 whose metamorphosis is actually 

 abbreviated in such a way that they 

 leave the egg at a later ontogenetic 

 stage. BARRANDE classes under 

 this type the following genera : 

 Arethusina, Cyphaspis, Proetus, 

 Arionellus, Conocephalites, Aeglina, 

 Hydrocephalus, lllaenus, Acidaspis, 

 Ampyx, Ogygia, and Triarthrus. 



A still more abbreviated meta- 

 morphosis seems to characterise 



FIG. 152. Three stages in the development 

 of Trinucleus ornatus (after BARRANDE). 



A, youngest stage, consisting merely of 

 the cephalic shield and the pygidium. 



B, stage with one free thoracic seg- 

 ment. C, stage with four free thoracic 

 segments. 



