Miscellaneous. 153 



continues slightly gaping for some time, after which it separates itself 

 entirely from all that characterized it in its early state, to become 

 irregular like the opposite valve, detaching itself entirely from its early 

 state (the two valves of its early existence), which remain attached 

 to the apical part of the inferior valves only. 



" In the adult age the shell is irregular, very variable in its form, 

 and very inequivalve. The inferior valve is fixed by means of its 

 substance, or rather filling and levelling all the inequalities of the 

 substance on which it rests, so as to solder it there, without however 

 in any part losing its external epidermis. It is generally oblong, 

 rounded behind, ending in a more or less long and irregular talon, 

 always roughened by the lines of growth ; the two anodontiform valves 

 of the young age are invariably found at the tip ; the interior is un- 

 equal, with a single subcentral muscular scar." 

 The animal of this shell has not been observed. 

 M. D'Orbigny compares the shell with the genera Etheria and 

 Ostrea, but by a very- extraordinary oversight does not make any 

 comparison of it with the genus Miilleria of Ferussac, with which it 

 agrees in every character, except that the specimens of that genus 

 described by Ferussac and Sowerby appear to have been imperfect 

 at the tip, so as not to exhibit the " anodontiform valves." 



M. D'Orbigny describes the adult shell as fixed by the right 

 valve ; but his figure represents it as fixed by the same valve as 

 the attached valve in the genera Miilleria and Etheria, which we 

 have always called the left one, because it is on the left side of the 

 animal when it walks. Ferussac properly described Etheria as 

 attached by its left valve, observing that it is rarely fixed by the 

 other. 



The regular form and free state of the young shell is not a peculi- 

 arity of this genus, but is common to all the attached Mollusca ; but 

 in this genus the young animals appear to continue free for a longer 

 period than is usual ; yet some bivalves, as Hinnites for example, 

 which eventually becomes attached, often remain free for a much 

 longer period ; indeed, until it has reached nearly its adult size. The 

 great peculiarity of this genus is the fact of the upper or free valves 

 becoming separated from the apical or young state of the shell ; but 

 this is easily explained when we consider that the first time the 

 animal opened this valve after it became attached, it must break the 

 valve away from the fixed apex. 



M. D'Orbigny lays great stress on the fact that the young shell has 

 two muscular scars, and the adult only one ; hence in its young state it 

 would be a Dimyaire, and in its adult one a Monomyaire of Lamarck 

 But it is to be remarked, first, that this idea has not been verified, for 

 the shell has not been seen in its young state, and the two muscles 

 are only supposed to exist because of the external forms of the young 

 shells ; and secondly, if it is true, it only teaches us what Nature is 

 constantly teaching us — not to place too exclusive reliance on a single 

 character, however general. While it does so, it does not in the slightest 

 degree militate against the value of the character derived from the 

 size or number of the muscular scars to distinguish the families and 



