10 BRITISH MARINE TESTACEOUS MOLLUSCA : 



out the hopeless attempt of a rigid, natural, precessioiial and 

 sequential progression. 



We almost think these facts and views go far to determine 

 that the true progressive course of the essential order of 

 nature of the class Mollusca, indeed of every other distinct 

 class of animated nature, is within our reach, and that what 

 we look for, depends on the more or less perfection of the 

 reproductive system in each well-characterized group of the 

 four grand types of the animal kingdom, in all which, if 

 closely examined, we think the progressive advance of gene- 

 rative organization is sufficiently apparent. 



ANALYSIS OF THE SYNOPSIS. 

 First Division. 



ACEPHALA PALLIOBRANCHIATA. 



I have removed this section of the Acephala from its posi- 

 tion at the head of the bivalves, to which I think it has no 

 pretensions. I consider it a distinct inferior group forming 

 the passage from the Ascidise and Cirripoda to the Acephala 

 lamellibranchiata ; by its pallial branchiae it has relations with 

 the Ascidise, and with the Cirripoda through the long convo- 

 luted cilial buccal appendages, which, though not articulated, 

 in consequence of advanced aiiimality, still prove its connec- 

 tion with that tribe. If the Palliobranchiata have the sexes 

 distinct, as some authors have stated, the position I now place 

 them in, with the strict hermaphrodite Acephala, would not 

 be correct, and in harmony with my sexual distribution ; but 

 I believe that these views of bisexuality in the bivalves are 

 erroneous, and the causes that have led to them are those 

 mentioned in the anatomy of Pholas dactylus under the head 

 of the "reproductive organs." 



The Brachiopoda are very rare British productions : I have 

 only met on the southern coasts with the minute Argiope 

 cistellula ; but the Terebratula caput serpentis and the Crania 

 anomala have been taken in North Britain sufficiently plen- 

 tiful to determine their anatomical structure. I refer for an 



