BIVALVIA. 



209 



adductors are rather small and deep, and the mantle-mark has an irregular sinus, by- 

 no means large. The shell is beautifully nacreous within, and the exterior is 

 ornamented with papillae or granulations, studded somewhat like the barrel of a 

 musical box. 



In the living state this species has been met with in deep water, while some of its 

 associates in the Coralline Crag are very shallow-water forms. Mr. Jeffreys has 

 dredged it off the Isle of Skye in 50, and Professor E. Forbes obtained it in the .ZEgean 

 at the depth of 150 fathoms. 



At page 148 (ante), I had supposed the genus Thetis to have been nearly related 

 to Lucinopsis, but this allocation is probably incorrect, In Mr. Woodward's ' Rudi- 

 mentary Treatise of Recent and Fossil Shells,* it is arranged in his family Myacida. 

 I think, however, the present species, Poromya granulata, cannot be correctly placed, 

 as it is there, between the genera Mya and Panopea. 



Corbula gigantea, J. Sowerby, Thetis gigantea, Woodward, has a granulated exterior, 

 with an external ligament, and faint or obsolete costse ; and if it be not a true Phola- 

 domya, it forms a connecting link between that genus and Thetis. 



Pandora,* Brug. 1792. 



HypogjEa and Hypogeoderma (sp.) Poli. 

 Tellina (sp.) Linn. 

 Solen (sp.) Mont. 



Calopodium. Bolten, 1 798. 

 Trutina. Brown, 1827. 



Generic Character. Shell transverse, inequivalve, inequilateral, ovate or subrhom- 

 boidal, externally smooth and of a nacreous texture, gaping at the anterior extremity, 

 one valve flat, the other more or less convex. Hinge with a prominent obtuse tooth 

 upon the right or flatter valve, and a corresponding depression for its reception in the 

 opposite one. Impressions of the adductor muscles subcircular, with a small or scarcely 

 perceptible sinus in that by the mantle. Ligament internal. 



The mantle is described as nearly closed, with a small passage for a narrow tongue- 

 shaped foot ; and the siphons are represented as very short, united nearly to their 

 orifices, which are fringed, and diverging. 



The inequality of the valves and internal ligament have been considered as charac- 

 ters sufficient to approximate this genus to that of Corbula, from which, however, it is 

 sufficiently removed, as essential differences exist in the animal inhabitant, but more 

 especially in the composition of its shell. In the examination and report by Dr. 



* Etym. Pandora, a proper name. This was given also to the inequivalved Pectens, probably from 

 their box-like character. 



