PLAGIOSTOMA, 



It appears probable tbat when living the Plagiostomata 

 were affixed by a byssus fixed to the animal, and passing 

 through the anterior part of the shell, which is usually 

 rather open. 



The Plagiostomata are inequilateral, slightly inequi- 

 valve, oblique shells, with small ears 5 they are for the 

 most part rather higher than they are long, and they are 

 covered with grooves or stri* diverging from the unibones 

 and passing to the ventral margin. In one species the ribs 

 between these grooves are here and there ornamented with 

 a longer or shorter spine. 



