400 Dr. J. E. Gray on some Families of Bivalve'^ BhelU. 



the animal and shell with attention can sanction such a sepa- 

 ration. Messrs. Forbes and Hanley in the same manner sepa- 

 rate Venerupes {Irus) from Tapes for this reason, placing the 

 former in Gastrocluenidce and the latter in Venerida. The adhe- 

 sion of the mantle leaves also varies in degree in some genera, like 

 Lutraria, where they are united when the animal is alive ; but 

 separate easily from each other after death or when the animal 

 is preserved in spirits, which is not the case with Myadce and 

 other families. 



It is to be observed, that the animals which live permanently 

 in holes in rocks, as Pholadidce, Gastrochcenidce, Saxicavid^e, or 

 live nearly permanently sunk in the sand on the sea-coast, as 

 SolenidcB, Myadce, Saxicavidce, Corbuladce, Pandoridce, Soleno- 

 myadce, Lasiadce, or in the sand of rivers, as Glauconomida, have 

 the mantle-lobes united, leaving only a greater or smaller aper- 

 ture in front for the passage of the foot. 



Secondly, the animals which live sometimes sunk in the 

 sand, and at others move about on the shore, have the lobes free 

 beneath for the more easy movement of the foot, as the Vene-^ 

 rid<s, TellinidcE, Mactradce, Cyprinadte, Cardiadce, Corbiculidce, &c. 

 But the species of those animals which live most in the sand or 

 in holes, as the Venerupes of Veneridte, and Lutraria of Mac- 

 trades, and the Petricolce, have the mantle- lobes more united like 

 those of the former group, which they most resemble in habits. 

 Tapes and Venerupes of the latter group, and some of the former, 

 as Saxicava, are often provided with a beard by which they at- 

 tach themselves to the rocks, when they have been ejected from 

 their holes, or have been cast on places where they cannot form a 

 cavity for their protection. 



Lastly, the genera which affix themselves to marine bodies by 

 a byssus passing out between these valves, or by the outer sm*- 

 face of the valves themselves, so that they are always exposed, 

 or which live in holes in coral, like Pedum, or sunk head down 

 in the sand, like Pinna, with the shell wide open at the top, 

 have the mantle-lobes generally free all round, except at the car- 

 dinal, and sometimes on the hinder edge. 



The only exceptions to these observations which have occui'red 

 to me are in the genera Chamostrea and Myochama, abnormal 

 forms of Anatinida, which are attached by the outer surface of 

 one of the valves, and the two very anomalous families of Tridac- 

 nidce and Dreissenidce, both living attached by a beard to other 

 bodies, and generally more or less entirely exposed like the last 

 group, but they have the mantle-lobes united like the families 

 of the first category. 



Lamarck, Messrs. Forbes and Hanley and others have arranged 

 the families of Bivalves according to the adhesion and separation 



\ 



