LAMELLIBEANCHIATA OF THE LOWEE MARLS. 181 



Formation and locality. — In the soft micaceous clays below the Lower 

 Marls at Haddonfield, New Jersey. Collection of the Academy of Natural 

 Sciences of Philadelphia. 



SAXICAVID^. 



Genus PAKOPEA Menard. 



Panopea decisa. 

 Plate XXiy, Figs. 5-8. 



Panopea decisa Conrad. J. A. K. Sci., Phil., Vol. II, 2d ser., p. 215, PL XXIY, Fig. 



19. Meek, Check-list Smith. Inst., p. 15. 

 Glycimeris decisa (Con.) Gabb. Synop., p. 125. Meek, Geol.Eept. N. Jersey, 1868, p. 



727. 



Shell moderately large and ventricose, with moderately large project- 

 ing beaks, which are situated a little nearest the anterior end, widely gaping 

 at the posterior end and closed anteriorly. Anterior extremity rounded, 

 longest below the middle, anterior end truncated, projecting near the cardi- 

 nal line and receding below. Surface of the shell marked by very strong, 

 broad, concentric undulations most strongly developed on the middle of the 

 valves and becoming nearly obsolete on some specimens both anteriorly and 

 posteriorly. The valves are also often depressed along the posterior umbo- 

 nal slope, showing a distinct furrow at the bending of the undulations of 

 the surface at this point. 



The internal features of the species are not easily made out from the 

 imperfect casts under examination, the shell having been too fragile to leave 

 the impressions of pallial line or muscular scars so as to be traced with any 

 degree of certainty. The hinge, however, has been considerably thickened 

 and has left the imprint of its features on some of the specimens, so that by 

 the use of gutta-percha its features have been fairly shown. There is posi- 

 tive evidence of only a single projecting tooth in each valve, which ha.s 

 been long and incurved, but showing that the shell was not indented or 

 excavated for their reception, as should be the case in Glycimeris, to which 

 genus Messrs. Gabb and Meek have referred it. In the right valve, how- 

 ever, the hinge-fold recedes on the anterior side of the tooth, but not in the 



