66 



In this place I shall describe two species that 

 have a slight relation to the subject ; viz. a tri- 

 valve shell and a mytilus from Louisiana. I have 

 added also some species that have been omitted 

 in their proper places, or discovered during the 

 progress of the work. 



Twelfth Genus TREMESIA. 



Shell tri-valve, inequivalve, principal valve 

 patelloid, perforated in the centre ; the small 

 valve closing this aperture, as an operculum f 

 third valve inferior, lateral; the animal cepha- 

 lous, the head extensible by the medial opening, 

 with two lateral eyes ; no tentacula. 



This singular genus appears to be the type of 

 a new family intermediate between the Bra- 

 chiopes, Teredaria, and Patellaria; like the 

 teredaria it has three valves, but a head like 

 the patellaria, and this oculated and not tentacu- 

 lated head is central instead of being terminal. 



