Paliidestrina Jcnkinsi, Snu't/i, and P. ventrosa, Mont. 377 



evident. Tlie transverse rows of teetli are slightly more 

 arched in P. ventrosa than they are in P. Jenkinsi ; in the 

 former, moreover, the admedian teeth alternate with and 

 project slightly between the central ones, whereas in the latter 

 they are nearly in a line with, and stand clear of, the median 

 teeth, so that the whole radida has the appearance of being 

 more sharj)ly divided into longitudinal areas. The contrast 

 between the respective median teeth is yet greater. In P. 

 ventrosa the central cusp of the rachidian is as long as half 

 the width of the tooth, and is flanked on either side by three 

 others, whilst the single basal denticle on each side is barely 

 visible ; in P. Jenkinsi, on the other hand, the length of the 

 central cusp is not more than one third the width of the whole 

 tooth ; four or even five minor cusps flank this central one to 

 right and left of it, and four basal denticles stand out conspi- 

 cuously on either hand below the crest of the tooth. 



Portion of Eadula of: — A. Paludestrina Jenkinsi, Smitli 

 B. Palude-^trina ventrosa, Mont. 



Two rows of teetli are in each case shown on the left of the median 

 tooth in their undisturbed position. On the right the teeth of a single 

 row are di'awn apart. 



The remaining difl'erences are best seen from the figures. 

 The following brief descriptions, with approximate micro- 

 measurements, of the radulffi of these two species are founded 

 on several specimens of each. 



