BIVALVIA. 267 
Poromya, Lorées, 1843. 
CorBwta (sp.) Nyst and West., 1839. 
Ema. Lovén, 1846. 
Generic Character. ‘Shell ovate or suborbicular, equivalve, inequilateral, slightly 
produced posteriorly ; surface invested with a scabrous epidermis, beneath which it is 
pearly and minutely punctated; hinge of a minute cardinal ossicle or erect tooth in 
one valve, lodged in a pit, or rather depression in the other; no lateral teeth ; ligament 
external ; palleal impression very slightly sinuated. 
“Animal with its mantle open in front; foot long, narrow, and slender; siphons 
short, unequal, with 18 or 20 tentacles surrounding their bases.”—ord. and Haznl. 
“ Testa equivalvis, posticée hians, truncaté ; ligamentum internum fovee utriusque valve 
insertum, ante quam in v.d. dens cardinalis, in v. s. fossa cardinalis ; in v. s. dens lateralis 
anticus et posticus ; in v. d. fossa lateralis, dentes laterales nulli. Impressio palliaris lata, 
duplicata, postice leviter sinuosa. 
** Animal pallio ventre aperto, posticé longe cirrigero, siphonibus instructum.’’—Lovén. 
Genus, Hmbla. 
Mr. Woodward, in his ‘Rudimentary Treatise of Recent and Fossil Shells,’ has 
concluded the Poromya of Forbes to be a species only of the genus Thetis. 
The establishment of a genus by the above-named eminent and able modern authors 
upon an existing shell, the one describing it as possessing an external ligament, while 
the other considers it to have an internal one, leaves it, as it were, a sort of open 
question, or placing it rather in a doubtful position. The type of the genus Thetis has 
an external ligament, whereas in the recent British shell and Crag fossil the hinge 
furniture is more complex; and although a portion of the ligament might have been 
seen externally when the valves were closed, the larger or cartilaginous part was 
situated within the edge of the shell, and its action like that of an internal ligament, 
opening the valves by expansion on the removal of pressure; no portion of which 
internal ligament appears to be present in those fossils constituting the genus Thetis ; 
and as I am imposing no new name for the Crag shell, the correct position must be 
determined by better materials than I possess; though, judging from my own speci- 
mens, I am inclined to believe with Professor Loven, that the action of its ligament 
was that of an internal one; and although the greensand fossils are no doubt closely 
related, the difference in position or action of the ligament is sufficient to justify the 
separation.* 
* The boundary line of generic isolation is indeed exceedingly difficult to define. We all of us give 
what we conceive to be a limit, but the want of accordance in this respect shows at least that we are as yet 
very far from having discovered it. The different positions of the ligament in Bivalves, whether acting 
internally by compression and dilatation, or externally by contraction and elongation over a fulcrum, are 
distinctions as good as nine out of ten of the characters that are generally employed for these conventional 
divisions, 
