PARALLELODON. 
39 
III. Family. — AROiDiE, Oraij, 1840. 
I. Suh-family. — ARCiNyE, H. and A. Adams, 1858. 
1. Genus. — Paeallelodon, Meeh and Worthen, 1866. 
The palgeozoic shells ranged under this genus correspond in shape with the 
Jurassic species for which Lycett founded the genus Macrodon in 1845, and only 
differ by having the anterior teeth transverse instead of oblique. The two genera 
are united by Zittel and by Fischer, but de Koniuck doubts their identity. 
Under any circumstances Lycett's name cannot stand, as it was preoccupied 
for a group of fishes by J. Miiller in 1842. 
1. Parallelodon atavds, n. sp. PI. I, figs. 1, 1 a-. 
Description. — Left valve tumid, very long and narrow, almost trapezoidal. 
Umbo situated almost at the anterior extremity, low, flattened, and turned 
forward. Area apparently rather broad and concave. Hinge-line straight, 
almost the length of the shell. Anterior margin rounded, obtuse. Inferior 
margin nearly straight, slightly oblique. Posterior margin oblique above, 
rounded at the postero-inferior corner. A strong angle running from the umbo 
to the postero-inferior corner. Hind wing flattened. Inferior edges so convex as 
to meet almost at two right angles. Surface marked with numerous fine, 
parallel, well-defined, raised, and rather distant threads, following the course of 
the margins and ending perpendicularly to the hinge-margin. Shell-structure 
rather thick. 
Size of left valve. — Length 21 mm., breadth 10 mm., depth 5 mm. 
Locality. — A single valve from Wolborough is in Mr. Vicary's Collection. (/3.A/ 
Bemarhs. — This shell evidently belongs to a very distinct species, but the 
single specimen with which I am acquainted is insufficient to allow as complete a 
definition as could be wished. The anterior margin, the hinge, and the right valve 
are unseen ; but the general character of the shell is clear enough. It seems 
probable that the anterior part of the inferior margin was somewhat gaping. 
Mr. Vicary's specimen has been labelled " Sanguinolites.'' I was at first 
doubtful whether to adopt this view, but the thickness of the shell-structure and 
the prominence of the umbo seem to be arguments against it. On the other hand, 
