76 THE DOCTRINE OF DESCENT. 



Turrilites, Scaphites, Baculites, and others are considered. 

 The prime of the great sea-Hzards is also past, but the 

 marshes of the Wealden period harboured new forms 

 of colossal land-lizards. The long-tailed cray-fishes are 

 joined by the true crabs, the most highly dev^eloped 

 forms of the class. In the Oolite and Chalk also occur 

 the chief of the sea-urchin-like Echinoderms. As yet we 

 have not mentioned the class of Echinodermata, in order 

 that we might here point out in conjunction several of 

 the more important phases of their geological occurrence. 

 Desor,^' a distinguished judge of this class, has lately 

 examined how in this large group of Echinse the pro- 

 gress of organization is gradually manifested, on which 

 occasion he was induced to make some general reflec- 

 tions on the principle of progression, as applied to 

 the Echinoderms, probably known to all our readers in 

 their representatives the star-fish and sea-urchins. If 

 articulate, as well as vertebrate, animals attain a higher 

 grade of development by the differentiation of the con- 

 secutive segments of the body, the superior unity, and 

 therewith higher perfection, of the Echinoderm's body 

 is evinced when the spines, or so-called antimera, give 

 way to the unity of the whole. 



The more distinct these elements are, that is to say, the 

 more independent they remain, the lower is, not only the 

 articulate animal, but also the Echinoderm. Accord- 

 ingly, the star-fish, and to some extent the feather-stars, 

 stone-lilies, or crinoids, occupy the lowest rank But 

 here, unluckily, palseontological tradition likewise aban- 

 dons us. Only so much is certain, that in the older 

 fossiliferous strata both divisions are abundantly repre- 



* Bulletin de la Societe des Sciences Naturelles de Neufchatel, IX. 2. 



