PROOFS, ILLUSTRATIONS, AUTHORITIES, ETC. xix 



of Acephala in the series of formations. Of all modern palaeontolo- 

 gists, M. de Buch is the individual who has studied the Brachiopods 

 with the greatest care ; and it is to his works above all others that I 

 refer for the detailed 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, ibr the most part, 

 to the order of Dimyaires. To make up for this, the intermediate 

 formations afford a remarkable assemblage of Brachiopods, Mono- 

 myaires, and Dimyaires, 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 

 Monom} 7 aires and Brachiopods which, at the era when they existed 

 alone, gave to the acephalous faunas the singular character of want 

 of symmetry in the sides, combined with a very remarkable sym- 

 metry before and behind." Agassiz : Monographic des Poissons 

 Fossiles. 



Gasteropoda. " The absence even in the coal-measures of land - 

 shells is a singular, and if I mistake not, a significant fact. The 

 known living species of the genera Helix, Cyclostoma, Bulimus, 

 Achatina, Pupa, and Clausilia, exceed 2000 in number ; and not one 

 of these genera, nor any of the pulmouiferous mollusca, such as 

 Lymnseus, Planorbis, Physa, etc., have as yet been detected in any 

 one of the primary strata from the Silurian to the Permian inclusive. 

 . . . Some few shells of the coal-measures have been referred to the 

 genus Unio, and others to an annelid allied to Spinorbis, and called 

 Microconchus, probably an inhabitant of brackish water." Sir 

 Charles Lyell, 1851. 



Fresh-water mollusca of the genera Cyclas and Unio have been 

 found in the marl-stones and shales of the Lias in Gloucestershire 

 and other parts of the West of England. 



Planorbis and Lymnseus, as well as Valvata, Physa, and Melauia 

 in abundance (pulnioniferous mollusca), were found by Professor 

 E. Forbes in the Purbeck beds, which are supposed to be of the 

 Oolite age. 



No helices yet detected so early. If I ma}- take Bartlett's Index 

 Geologicus as a guide, there seem to be no land shells till the post- 

 pleistocene era. 



The organic gradation of the Gasteropoda is precisely in this order. 

 " What," says Professor Agassiz, " is the respective position of the 

 marine families, of the fluviatile families, and of the terrestrial fami- 

 lies ? The natural gradation established by their structure, 



agrees with their natural connexion with the elements in which they 

 live, in the order which I have assigned to these, the types of Gaste- 

 ropoda which are lowest being exclusively marine, the highest 



