Bi-itish Marine Testaceous Mollusca. 475 



been taken in North Britain sufficiently plentiful to determine 

 their anatomical structure. I refer for an account of the animal 

 of the T. caput serpentis to that invaluable work the ' British 

 Mollusca/ and for other general observations to Professor Owen's 

 paper on this family in the first volume of the ' Zoological Trans- 

 actions/ - 

 Ylosifil i9il • Second DivisioN;ini has easiBcaoas moil 



Acephala lamellibranchiata. 



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 con- 

 curring in this opinion. 



The anatomy of the internal organs of the entire tribe as to 

 generalities is so similar, that it scarcely affords sufficiently de- 

 cided generic distributive points ; my anatomies of Pholas dac- 

 tijlus and Teredo megotara confirm this position ; I have therefore 

 had recom-se 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 at- 

 tempted on ligamental bases of internal or external position ; but 

 it has been found so unstable and arbitrary, that if strictly fol- 

 lowed, the most incongruous species would be associated : for in- 

 stance, Mactra solida would march with Anatina prcetenuis, and 

 the Cardia with the Saxicavse. The disposition of the adductor 

 muscles has been tried, and appears to be delusive and unsatis- 

 factory, as most, if not all, Lamarck's Monomyse have two ad- 

 ductor muscles, though the volume of one is much greater than 

 the other. We think the only true Monomyse are the Pholadidae, 

 as we have shown in the memoir on the anatomy, and these are 

 Dimyse 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 aids is to throw the 

 tribes, genera, and species together by the best mode that agrees 

 with their composition. 



It has long been the fashion, without any particular good rea- 

 son, for Pholas, Teredo, &c. to commence the Acephala, and to 

 terminate them with the Pectines, Ostrece and Anomice, &c. I 

 admit, as regards the essential points of natural order, that it is 

 not very material whether Pholas and Teredo stand first or last 

 in the scale. But in the classification I have adopted, which is 

 founded on the progressive advancement of the reproductive or- 



