Acephala. 25 



its sides, and the sides have very generally and erroneously 

 been regarded as the dorsal and ventral regions. The ante- 

 rior and posterior extremities, on the contrary, are shaped 

 with the most perfect symmetry ; that is to say, in other 

 words, the front and the hinder part of the animal cannot 

 be distinguished, while its sides shew a marked difference. 

 In the Monomyaires in general, and among the Ostracea in 

 particular, we observe a conformation intermediate between 

 that of the Brachiopods and that of the Dimyaires ; the sides 

 are still very different, but now one of the edges appears as 

 the anterior extremity of the body, and the animal, still ad- 

 hering in the case of oysters, has no longer, in all the genera, 

 the absolutely lateral position of the inferior types ; wit- 

 ness the Pectens, which swim freely. Lastly, among the 

 Dimyaires, the bilateral symmetry attains to full perfection, 

 and, at the same time, one of the extremities of the body is 

 sensibly characterized as the anterior. The animal then as- 

 sumes a position more or less vertical, the head in advance, 

 and the relation of its organs with the surrounding media are 

 analogous to those of other symmetrical animals. 



These connections are fully justified by the order of the 

 succession of the Acephala in the series of formations. Of 

 all modern palaeontologists, M. de Buch is the individual who 

 has studied the fossil Brachiopods with the greatest care ; 

 and it is to his works above all others that I refer for the de- 

 tailed study of the facts, the principal results of which I am 

 about briefly to state. In the most ancient formations, we find 

 nothing but Brachiopods, but in such profusion, and in forms 

 so varied, that in their abundance and diversity, they scarcely 

 yield to the Acephala of the tertiary formations, in which the 

 brachiopods have almost entirely disappeared, to be replaced 

 by an innumerable quantity of species of different genera, 

 belonging, for the most part, to the order of Dimyaires. To 

 make up for this, the intermediate formations afford a re- 

 markable assemblage of Brachiopods, Monomyaires, and Dim- 

 yaires, the more interesting from this, that the Dimyaires 

 with non-symmetrical sides still exceed in number those 

 which are perfectly regular, and thus become connected with 

 the Monomyaires and Brachiopods which, at the era when 



