BIVALVIA. 267 



POROMYA, Forbes, 1843. 



CORBULA (sp.) Nyst aud West., 1839. 

 EMBLA. Lovtn, 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." Forb. and Hani. 



" Testa tequivaluis, postice hians, truncatd ; ligamentum internum/by& utriusque valvce 

 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, denies laterales nulli. Impressio pattiaris lata, 

 duplicate) postice leviter sinuosa. 



" Animal pallio venire aperto, postice longe cirrigero, siphonibus instructum." Loven. 

 Genus, Embla. 



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. 



