52 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



part of the hinge-plate is smooth and vertical as in the right valve, and slightly 

 bevelled from below upwards. 



The form (5, PI. V, figs. 17 — 34, has lost the simple character of right-valve 

 tooth anterior, socket posterior, and left-valve socket anterior, tooth posterior, 

 and has in the right valve its anterior portion much bevelled at the expense of its 

 lower border ; while the upper border of the hinge-plate is raised into a long 

 irregular tooth, which extends considerably behind the umbo, and has, posterior 

 to it, a narrow wedge-shaped depression, with apex upwards, to receive the tooth 

 of the opposite valve. 



The posterior part of the hinge-plate is as in the form described above. The 

 great feature here is the absence of the broad depression in the right valve below 

 the umbo, and in the left valve anterior to it. 



In the left valve, anteriorly, the lower part of the hinge-plate is produced, 

 while its upper border is reduced, and becomes hollowed longitudinally to receive 

 the elongated tooth of the right valve, behind which is an elevated collar sloping 

 backwards, against which the tooth in the right valve rests. 



The posterior part of the hinge-plate is rarely a plain surface ; at times it is 

 twisted on itself, and may be excavated in places, as in recent Unios, for the 

 attachment of the posterior extremity of the ligament. The form of lock is 

 vertical, the tooth of right valve being superior to that of the left. 



It has, I think, from the observation of transitional forms, been evolved by 

 the elongation backwards of the tooth of the right valve, encroaching on the con- 

 cavity for that tooth in the left, causing a corresponding atrophy of that tooth ; 

 at the same time, a bevelling of the lower part of the anterior portion of the 

 hinge-plate of the right valve, and a corresponding increase in the lower part of 

 the anterior portion of the left, took place. 



The third form, y, PI. V, fig. 37, differs from the type /3 in having in the right 

 valve the one long cardinal tooth bifid, with a triangular pit between the two 

 portions. The left one has a single median cardinal tooth, with a concavity in 

 front and behind it. The vertical locking arrangement, by bevelling the edges of 

 the hinge-plates of the respective valves, is as in form /3, so that in form y a much 

 stronger lock is obtained, having both a vertical and horizontal articulation. 

 Occasionally I have met with a transposition of the hinge arrangement, the 

 cardinal tooth then being anterior, and overlapping that of the right (PI. V, fig. 7). 



Dimensions : 



Antero-posteriorly ... 54 mm. 



Dorso-ventrally ... 28 mm. 



Laterally .... 20 mm. 



Localities. — England : Yard-coal, Chesterton ; Cockshead Coal-roof, Adderley 

 Green; Ten-foot Coal, Bucknall; Banbury Seam, Bucknall; Knowles Bass,Longton; 



