318 SYSTEMATIC PALEONTOLOGY 



Description. — " Cordate-globose, slightly oblique, with rather large 

 concentric wrinkles, and lines of growth; an elevated undulation on the 

 anterior submargin, marking the greatest length of the shell; umhones not 

 very prominent, apex rather suddenly incurved, acute; impressed space 

 behind the beaks, dilated and rather profound; anterior tooth striated 

 externally, and placed on the middle of the anterior margin." Say, 1824. 



Say had the anterior and posterior ends transposed so that for each 

 reference to direction in the above description the opposite direction is to 

 be understood. From the drawing and description. Say's large specimen 

 was very probably a Virginia form. Specimens from Maryland are 

 smaller and less rounded and have a more pronounced ridge and a basal 

 angle where the dorsal and posterior slopes and margins meet. These dif- 

 ferences seem constant but are not deemed of sufficient importance to jus- 

 tify separating the Maryland forms from those from Virginia. 



Length, 73 mm. ; height, 59 mm. ; diameter, 29 mm. 



Occurrence. — St. Mary's Formation. Cove Point (?), St. Mary's 

 Eiver. Choptank Formation. Governor Eun, 2 miles south of Gov- 

 ernor Run, Flag Pond, Jones Wharf, Pawpaw Point. Calvert Forma- 

 tion. Plum Point. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



ISOCARDIA IGNOLEA n. Sp. 



Plate LXXXV, Figs. 1, 2. 



Description. — Shell oval, moderately elevated anteriorly, gently de- 

 pressed posteriorly ; beak depressed, moderately incurved ; surface of shell 

 with numerous gentle, somewhat irregular, close-set, concentric undula- 

 tions most prominent on the marginal two-thirds of the surface ; meeting 

 of posterior and umbonal slopes marked by a ridge, of posterior and basal 

 margins by an angle ; posterior margin bluntly rounded ; a cardinal and a 

 posterior lateral tooth in left valve, two cardinals in right valve ; ligament 

 area curved, ridged, and grooved; interior smooth; muscle impressions 

 and pallial margin distinct. 



It is unfortunate that the locality from which this species comes is in 

 some doubt. The only specimens — the two valves of the same indi- 

 vidual — were found in a case of University material from Cove Point, but 

 the color of the weathering, state of preservation, and incrusting material 



