The external characters of Chonetes comoides have been carefully described by several authors†, but little or nothing appears to have been written on its internal arrangements. Sowerby's figure does not convey a correct idea of the double area, fissure, and cardinal process belonging to this species; he simply represents a bivalve shell possessing a convex and concave valve, finely striated, with a flat parallel space between the beaks. Better illustrations were, however, subsequently published by Count Keyserling, and reproduced by M. de Koninck: therein the characters of the double area, cardinal process, and fissure are faithfully delineated ; but the recent discovery of several more perfect examples has made it desirable to publish a few observations and illustrations additional to those we already possess.
The Chonetes comoides may be deseribed as a large semicircular, inequivalve shell, with a straight hinge-line, generally (but not always) as long as the greatest width of the shell, and articulated by means of teeth and sockets. The ventral or dental valve is exteriorly more or less eonvex, with ear-shaped expansions. The dorsal or socket valve concave, following the curves of the other. The beak of the ventral valve is variably developed in different examples,—a circumstance due to the lesser or greater convexity of the shell : thus in specimens with a moderately convex valve the beak is rarely produced or visible above the cardinal edge, while in more convex individuals (as in the typical examples figured in the ‘Min. Con.’) the beak is more or