43 



up a communication with the surface by means of its 

 retractile tube, which is greatly elongated when pro- 

 duced to its utmost length. The inexperienced tra- 

 veller of the sands inhabited by this shell is surprised 

 to feel a jet of water sometimes ejected upon his legs, 

 which is caused by the animal^ when the sense of dan- 

 ger induces it suddenly to contract its tube. It is 

 known in the southern states by the name of Mani- 

 nose. It is occasionally found much larger than the 

 specimen figured. 



THRACIA. 



GENERIC CHARACTER. 



Shell thiu;, inequivalved^ ventricose, gaping poste- 

 riorly; hinge with a thick callus in each valve ; ligament 

 exterior, situated in a linear groove in the callus ; 

 muscular impressions two, the anterior one elongated; 

 palleal impression with a deep sinus. 



OBSERVATIONS. 



This genus was separated from Anatina by Dr. 

 Leach, and adopted by Blainville. I should suppose 

 from the habit of the shell, that it was more nearly 

 related to Mya than to any other genus, but unlike 

 Mya it inhabits the ocean exclusively, and the animal 

 appears to be unknown. 



