Maryland Geological Survey 575 



one over the other, the tree ventral edges being visible on the external 

 surface and their dorsal edges on the interior; radial sculpture absent 

 upon the left valve but often quite strongly developed upon the right ; 

 ligament area trigonal, the medial sulcus broad and shallow, indenting 

 the inner margin, area of the left valve strongly undercut, that of the 

 right valve truncated ; the ligament area of the two valves forming a 

 V-shaped trough, thus allowing the cover-valve to be opened quite widely ; 

 vermicular corrugations developed on either side of the ligament pit ; 

 muscle scar very distinct, even profound in some of the more ponderous 

 individuals, placed a little behind the median line and quite high up. 



Xo Upper Cretaceous group is more sadly in need of monographic 

 treatment than that of 67. reticularis (Lamarck), nor does any bear 

 promise of yielding more interesting results in general correlations. The 

 group is world-wide in its distribution — it has been reported from the 

 East Coast, the Gulf and the Western Interior of North America, Eng- 

 land, Central Europe, Eussia and Southern India, and is usually one of 

 the most prominent elements in the faunas in which it occurs. This 

 wealth of material from widely separated localities has made the problem 

 of separating geographic from chronologic influences an exceedingly deli- 

 cate one and a problem which demands for its proper solution a con- 

 sideration of all the types of variation at all the occurrences. Until this 

 can be done it seems wiser to avoid further confusing the literature by 

 frankly evading the question, merely indicating the lines of variation 

 followed by the Maryland Gryphseas and assigning to these subgroups 

 the non-committal name of " races." 



Five distinct races are present in Maryland, distinct in their peripheral 

 members but with intergrading individuals. 



Race A. Plate XXVIII ; Plate XXIX. Fig. 1 

 (A) Shells of moderate size, usually very heavy, the posterior dorsal 

 margin often produced and separated from the central disk by a broad 

 and shallow depression; the right valve not very much smaller than the 

 left and feebly or not at all concave. This is the G. mutabilis of Morton, 

 and is the race so exceedingly abundant along the Canal, particularly at 



