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 f, Pl. 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 hgament. 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, Pl. V, fig. 37, differs from the type B 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 (, 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 : : é 54 mm. 
Dorso-ventrally 5 : : 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; 
