SYNOPSIS OF GENERA. 
xlix 
basal extremity; post-cardinal slope often with an obscure or more or less 
defined fold along the middle of its area. 
Surface marked by more or less regular, fine concentric striae of growth. 
Hinge-line (in the cast) marked by a groove indicating a strong lateral tooth 
or fold. There is likewise a groove along the hinge-line anterior to the beak 
and a divergent furrow extending toward the anterior margin. Ligament 
external. Muscular impressions and pallial line undetermined. 
The species of this genus constitute a remarkable group of inequivalve shells. 
Their geological position is in the upper part of the Chemung group, and the 
only others which might be considered as having relations with them are the 
Palaeanatinae, which are similar in their inequivalve character. 
The hinge characters cannot be fully determined, but the remarkable feature 
of a cardinal line extending the entire length of the shell with a longitudinal 
fold, which seems scarcely to be interrupted at the beaks, together with the 
diverging anterior process or clavicle, are characteristic and peculiar features. 
Examples: Prorhynchus quadratum, pi. lxxix, figs. 40-49 ; pi. xcvi, fig. 5. 
Prorhynchus nasutum, pi. xcvi, fig. 6. 
Prorhynchus angulatum, pi. xcvi, fig. 7. 
Glossites, n. gen. [Type, Glossites lingualis, Hall.] 
Shell equivalve, inequilateral, narrowly or broadly elliptical in outline. An¬ 
terior end short, margin declining from the beak and curving below. Posterior 
end large, narrowly or broadly rounded at the extremity. Beaks small and ap- 
pressed, sub-anterior, or in advance of the anterior third. Cardinal line long, 
gently arcuate. Umbonal slope not defined, prominent above and usually 
merging into the general convexity of the surface about the middle of the length. 
Surface marked by fine concentric striae of growth, which are often raised 
into sharp sub-lamellose elevations, or more rarely by stronger concentric ridges. 
Obscure radiating striae are rarely observed. 
Hinge characters undetermined. There is evidence of an elongate lateral 
tooth or fold, and the valves are joined by a strong external ligament. Lunule 
distinct. Muscular impressions shallow. 
G 
