102 INTRODUCTION. 



Productus (part), of Pander (not of Sow). 

 Platystropiiia, King. 

 DlC(ELOSlA, King. 

 ScHizoPHORiA, King.^ 



Shell variable in shape, subcircular or quadrate ; valves equally or unequally convex ; 

 socket valve sometimes slightly concave, with or without a mesial fold or sinus ; hinge 

 line straight, generally shorter than the width of the shell ; both valves furnished with an 

 area divided by a triangular open fissure for the passage of the pedicle fibres ; beaks 

 more or less incurved, that of the larger valve generally more produced ; surface smooth, 

 striated, or ornamented by simple, bifurcated, or intercalated ribs ; structure minutely 

 or largely punctated; valves articulating by means of teeth and sockets. In the interior 

 of the larger or ventral valve the vertical dental plates form the walls of the fissure, 

 and extend from the beak to the bottom of the shell ; between these a small rounded 

 mesial ridge divides the muscular scars which extend over two elongated depressions 

 margined on their outer side by the prolonged bases of the dental plates ; the cardinal 

 muscles appear to have occupied the greater portion of the anterior division of these 

 two depressions, the pedicle nmscles occupying the external and posterior part of the 

 same space; the adductor was probably attached to each side and close to the mesial ridge. 

 In the socket valve the fissure is partially or entirely occupied by a more or less produced 

 simple shelly process,^ to which were affixed the cardinal muscular fibres ; the inner socket 

 walls are considerably prolonged into the cavity of the shell, under the shape of pro- 

 jecting laminae, to the extremity of which free fleshy spiral arms may, perhaps, have been 

 affixed. Under this shelly process a longitudinal ridge separates the quadruple impressions 

 of the adductor, which on each side forms two deep oval depressions, placed obliquely one 

 above the other, and separated by lateral ridges branching from the central one ; the 

 pallial vessels, as well as their numerous minor bifurcations or veins, have often left 

 impressions within the valves ; the principal trunks seem both more numerous and better 

 defined in the socket valve, — after extending in a somewhat radiate or sub-parallel direc- 

 tion from the muscular scars to near the anterior region, they sweep round sub-marginally 

 on both sides of the valve, leaving wide ovarian spaces, and giving off a series of smaller 

 branches. 



' I am requested by Professor King to state that from not having been acquainted with Dalman's 

 original work ' Petrefacta Suecana,' in the Proc. of the Royal Acad. Sc. of Stockholm, he had not per- 

 ceived that the author had placed marks of doubt after his first tM'o types of Orthis, otherwise he would 

 not have proposed the term Schizophoria, which he now considers as a synonym of Orthis, This mistake 

 he attributes to Hisiuger having omitted Dalman's marks of doubt. 



2 Some authors term the bifid or trifid cardinal process a " rostral tooth ;" but I strongly object to 

 such a denomination, since the only true teeth are those existing in the dental valve, and the shell does not 

 articulate by means of the bifid projection, which was in all probability destined, as in other genera where it 

 exists, for the attachment of the cardinal muscles. 



