586 CRETACEOUS PALEONTOLOGY. 



middle of the basal margin; postero-basal margin a little more 

 sharply rounded; posterior margin convex, a little straighter 

 than the anterior. Valves strongly convex or gibbous, most 

 prominent, but not at all angular, along an oblique line from the 

 beaks to the postero-basal extremity, the posterior slope more 

 abrupt than the anterior. Muscular impressions large, the an- 

 terior ones deeply impressed above, the posterior ones scarcely 

 differentiated from the general surface of the casts. Hinge char- 

 acters not seen. Inner free margins of the valves apparently 

 not crenate. Shell substance thick, rugose externally. The sur- 

 face markings consist of strongly elevated, rounded, radiating 

 costse, narrower than the interspaces ; on a specimen about 55 mm. 

 in length, the distance between these ribs from center to center 

 at the middle portion of the shell margin, is about 2 mm. or a 

 little less. Each third interspace is occupied by a row of strong 

 and thick spines rising one or two millimeters above the tops of 

 the costae when complete, subcircular in cross-section, their bases 

 occupying the entire width of the furrow, the space between suc- 

 cessive spines being about equal to the thickness of the spines 

 themselves; in some cases the bases of the spines are thickened 

 longitudinally so that they occupy essentially the entire furrow, 

 in which case the two bounding costae with the row of spines 

 rising from the intervening furrow, appear to form altogether, 

 one broad rib supporting a row of strong spines. The two fur- 

 rows intervening between the rows of strong spines are each 

 occupied by a row of very much smaller, laterally compressed 

 spines whose bases are more or less connected. 



Remarks. There is considerable variation shown in the sur- 

 face markings of different individuals of this species, and the 

 extremes might be taken as the representatives of distinct species 

 or even of distinct subgenera. In its typical form as seen in the 

 Tinton beds, the species exhibits clearly the characteristics of the 

 subgenus Criocardium, the rows of spines rising from the inter- 

 spaces between the radiating costse of the shell. In some speci- 

 mens the bases of the larger spines or nodes are confluent and 

 appear to entirely fill the interspace occupied by them, so that 

 the two bounding costae with the row of spines together seem to 



