9 GO^CBYLIA—DITHYEA. 1. 



Some of them, however, have the valves strengthened by acces- 

 sorial ones, placed about the hinge or in front ; and some are 

 merely united by a strong elastic membrane. 



These accessorial valves form no part of the necessary generic 

 character, as some of the species are entirely without them, and 

 in others they vary both in number and shape. 



From the Teredo this genus differs, both in the shape of the shell, 

 which is transversely elongated and oval, and in wanting the 

 cylindrical tube with its accessorial appendages. 



* Valvis sulco longitudinali divisis. 1-4. 

 The valves divided by a longitudinal groove. 



Papyracea. Pholas testd clavatd, latere antico clauso obtusissimo, postico hiante 

 truncato accessorio producto annulari. 

 Shell club-shaped, closed and very obtuse at the anterior end, 

 open and truncate at the other end, which is furnished with a 

 produced accessorial ring. 



Tab. nost. 1. fig. 1 to 4. 

 Pholas papyraceus. Solander, Manuscripts. 



Portland Catalogue, p. 82, lot 1828. 

 Pholadidea Loscombiana. Turton, Conch. Diet. p. 147. 

 Mus. nost. In Torbay. 



Shell three quarters of an inch long, and an inch and three quar- 

 ters wide including the ring, in color and appearance resembling 

 whitish brown paper: the anterior end inflated, very obtuse and 

 rounded, closed when the valves are shut, and quite smooth for 

 about half its syrface: the hinder half marked with close-set 



