434 mactrid^. 



and Hanley, and (with a single exception) all the Con- 

 tinental zoologists, I will not prolong the discussion, 

 beyond remarking that Lavignonus of Ferussac (1821), 

 Listera of Turton (1822), Abra of Risso from Leach's 

 MS. (1826), and Syndosmya of Re'cluz (1846) either 

 have never been clearly or adequately defined, or else 

 are superfluous. The last-named genus has indeed been 

 selected by Forbes and Hanley, and separated from 

 Scrobicularia, without stating any particular character 

 by which one can be distinguished from the other. 

 Clark at first considered them different, on the ground 

 that S. piper at a has but one gill or branchial plate on 

 each side, and that the palps are of a larger size in this 

 than in some other species. He subsequently modified 

 his opinion by suggesting that the single plate might 

 be double : the relative magnitude of the labial palps 

 can only be regarded by the mere malacologist as a 

 matter of anv importance. The sole diversitv between 

 the shells called Scrobicularia and Syndosmya consists 

 in the former having no lateral teeth, while the latter 

 possesses them in one or each valve. This character 

 notoriously varies in species of other genera, for in- 

 stance Tellina and T)onax. 



The circumstance of the tubes being separate in the 

 present genus, and united in Mactra and Lidraria, 

 might induce some conchologists to replace it among 

 the Tellinidcs : but the hinge is essentially Mactridan ; 

 and in Amphidesma, which has the same internal struc- 

 ture, the tubes are also disunited. I should distrust 

 any classification of families founded exclusively upon 

 the contiguity or remoteness from each other of these 

 outer folds or processes of the mantle called ' ' tubes " or 

 " siphons." 



