482 Mr. S. Stutchbury on Pachyodon, 



tooth in the left valve elongated and attenuated towards the umbo. 

 In the right valve there is a flattened fold parallel to the ligament, 

 and divided obliquely near the umbo by a faint groove ; from the an- 

 terior extremity of this fold a depression extends beneath the lunule 

 in front of the anterior lateral tooth, and in the left valve there is a 

 corresponding elevation. Umbones closely approximating; muscular 

 impressions deep ; ligament external, situated in a deep, marginal, 

 posterior dorsal sinus. 



This remarkable genus of bivalve shells differs essentially 

 from any other described, as will be seen by reference to the 

 generic character above. 



The difference between this and other genera, particularly 

 Unio, in which genus several species have hitherto been 

 placed, consists in the following characters : — In Pachyodon 

 the cardinal tooth is often obsolete ; the anterior lateral tooth 

 is thick, simple, without striae, situated in the right valve, ha- 

 ving for its insertion a corresponding hollow in the left valve ; 

 the posterior lateral tooth is placed in the left valve, the sul- 

 cus for its reception being in the right. There are two cica- 

 trices of muscular adhesion very deep and strongly marked, 

 with a small round impression above the anterior one in the 

 left valve. 



The impression of adhesion of the mantle is entire, having 

 no siphonal sinus. 



Shell thick and heavy, without any trace of erosion at the 

 umbo, so common to freshwater bivalves ; apparently without 

 nacre ; the lines of growth very prominent, forming strongly 

 marked transverse lines. 



1. Pachyodon Listeri, Plate IX. fig. 1, 2. 



Shell thick, heavy ; transverse lines of growth from yoth to ^th of 

 an inch apart, as distant near the umbo as towards the base ; height 

 from basal margin to the apex of the umbo, 1 inch y^ths ; length from 

 the anterior to the posterior margin, 2*0 ; thickness 0*7. 



Lias, Frethern, Gloucestershire, and Battledown, near Cheltenham. 



Unio Listeri, Sowerby, Min. Conch., tab. 154. fig. 1, 3, 4. 



. , Goldfuss, p. 181. pi. 132. fig. 1. 



Donux ? Parkinson, Org. Rem., vol. iii. pi. 13. fig. 7. 



Museum of Bristol Institution. 



2. Pachyodon hybridus, Plate IX. fig. 3, 4. 



Shell cuneiform, thick; lines of growth distant and strongly 

 marked; umbones closely approximating; lunule deep; height 12, 

 length 1*8, thickness 0*7. 



Lias, Langar, Nottinghamshire ; near Cheltenham, Gloucester- 

 shire. 



Unio hybrida, Sowerby, Min. Conch., tab. 154. fig. 2. 



Museum of Bristol Institution, Samuel Worsley, Esq. 



