THETIS. 367 



Genus THETIS, Sowerby. 



Siphons surrounded at their base by long cirrhi reflexed 

 on the shell. 



Shell ovate or sub-orbicular, ventricose, equivalve, slightly 

 produced and truncate posteriorly, valves covered with a 

 scabrous epidermis, beneath -which the surface is minutely 

 punctulated, internally slightly pearly. Hinge with a 

 single, erect cardinal tooth in the right valve, received 

 into a corresponding cardinal fossa in the left; no lateral 

 teeth in the right valve, but an anterior and posterior lateral 

 tooth in the left valve ; ligament external ; cartilage in- 

 ternal, inserted in a socket in each valve ; ossicle distinct. 

 Pallial line sub-marginal, posteriorly very slightly sinuated. 



Sy)i. Poromya, Forbes. Embla, Loven. Eucharis, 

 Recluz. 



Ex. T. granulata, Nyst and Westendorp, pi. 97, fig. 2. 

 Shell, T. granulata, fig. 2, a, 2, b. 



The genus Thetis, founded by Sowerby on a fossil species, 

 appears to be the same as the Poromya of Forbes, the Embla 

 of Loven, and the Eucharis of Eecluz. The shell figured 

 in the Zoology of the Voyage of H.M. S. Samarang as Po- 

 romya nitida is a species of Netera. The mantle of the 

 animal in this genus, as observed by M f Andrew, is open in 

 front for the passage of a slender, narrow foot, and the short 

 siphons are surrounded at their base by about eighteen or 

 twenty tentacular filaments which are reflexed over the hind 

 part of the shell. The species known are few in number, 

 and inhabit deep water ; they are from Norway, Britain, 

 China, and America. Fossil examples occur in the Neoco- 

 mian formations of Britain, Belgium, France, and Southern 

 India. 



