CLASSIFICATION. 1 1 



account of the animal of the T. caput serpentis to the f British 

 Mollusca/ and for other general observations to Professor 

 Owen's paper on this family in the first volume of the ' Zoo- 

 logical Transactions/ 



Second Division. 



ACEPHALA LAMELLIBRANCHIATA. 



The animals of this group are strict hermaphrodites, though 

 it is said that in some of them the sexes are distinct. We 

 dissent from this view, and have assigned in another place our 

 reasons for not concurring in this opinion. 



The anatomy of the internal organs of the entire tribe as 

 to generalities is so similar, that it scarcely affords sufficiently 

 decided generic distributive points ; my anatomies of Pholas 

 dactylus and Teredo megotara confirm this position ; I have 

 therefore had recourse to an arrangement which combines 

 both internal and external organs, to assist the distribution of 

 this numerous class into convenient groups. A divisional 

 order has been attempted on ligamental bases of internal or 

 external position ; but it has been found so unstable and arbi- 

 trary, that if strictly followed, the most incongruous species 

 would be associated : for instance, Mactra solida would march 

 with Anatina praetenuis, and the Cardia with the Saxicava. 

 The disposition of the adductor muscles has been tried, and 

 appears to be delusive and unsatisfactory, as most, if not all, 

 Lamarck's Monomya have two adductor muscles, though the 

 volume of one is much greater than the other. We think the 

 only true Monomya are the Pholadida, as we have shown in 

 the memoir on the anatomy, and these are Dimya with that 

 eminent zoologist. 



The teeth and foot as general guides are so variable as not 

 to be available ; the best of these aids is perhaps the greater 

 or less closure of the mantle; this last we have adopted. 

 It appears then that the animals cannot be allocated in a 

 continuous natural order with perfect satisfaction by any of 

 these modes : all that can be done by those who make use of 

 such assistance, is to throw the tribes, genera, and species 



