MOLLUSCA. 587 



constitute a single broad rib crowned with a row of strong nodes. 

 At the same time the rows of secondary nodes are sometimes 

 confluent at their bases and form a continuous secondary rib, 

 perhaps nodose on top, and about equaHng in height and size 

 the primary costse, so that there seem toi be three costse of nearly 

 equal size in the broad interspace between the rows of large nodes 

 and their included bounding costse. In the extreme development 

 of the rows of secondary nodes their bases are confluent and they 

 increase in size and height so as to occupy the whole of the in- 

 terspaces, obliterating entirely the primary costje, so that the 

 surface of the shell is apparently marked by radiating rows of 

 tubercles which apparently do not rise from interspaces between 

 costse, but directly from the surface, each third row being much 

 larger and stronger than the two intervening ones. 



It is possible that larger collections of more perfectly preserved 

 material than is now available, would show that more than one 

 species has been included under this head, but so far as can be 

 determined from present collections, all these forms seem to 

 run together. The typical form of the species, however, is that 

 in which the nodes rise distinctly from the interspaces, showing 

 the characters clearly of the subgenus Criocardium, and which 

 has been recognized only in the Tinton beds. 



In its somewhat elongate and slender form, the species in the 

 form of internal casts somewhat resembles the casts of C. 

 spillmmii and they have sometimes been so identified. It does 

 not grow so large as that species, however, it lacks the radiating 

 ribs usually impressed upon the posterior slope of C. perelon- 

 gatum, and the anterior muscular scar is not so low in position. 



In the collections of the National Museum at Washington this 

 species is represented by numerous examples from the South which 

 have usually been referred to Cardium dumosuni. These south- 

 ern specimens are perfectly preserved shells which are smaller 

 than the usual examples from the Tinton Beds in New Jersey, 

 but their surface markings are identical with those of the type 

 specimen. The species differs from C. dumosum in its more 

 elongate form and in the much coarser surface markings. C. 

 tippana is another allied form in which the surface markings 



