of the Costce in the Perisphinctince. 



459 



costa while the third is still attached. The fourth group 

 shows the strength in the third costa, one free and two 

 directed towards three. The costae at this point are in- 

 terrupted by a constriction beyond which the costal 

 arrangement instead of continuing the trend shown in 

 the preceding inter sphincter ial sector shows a retrogres- 

 sion. But the first triad of costae does not show the same 



Fig. i. 

 VEN TER 



APICAD 



3 « ,ORAD 



'/jwm 



DORSUM 



Fig. 1. — Arrangement of costae on last half of fifth volution of the 

 holotype of Perisphinctes cubanensis O 'Connell. S. constriction or sphincter 

 marking interruption in regular costal development. 



APICAD 



Fig. 2. 



VEN TER 



I Z 3 \ 2 3 \ Z 3 * 5" 6 



ORAD 



DORSUM 



Fig. 2. — Arrangement of costae in early part of sixth volution of the 

 holotype of Perisphinctes cubanensis O'Connell. The orad migration of 

 the strong costa is shown, as well as the mode of development of free 

 ' ' intercalated ' ' costae. 



arrangement as the first triad in the preceding sector; 

 it marks instead a stage intermediate between the first 

 and second groups. Throughout the second sector the 

 groups of three are slightly in advance of the same 

 groups in the first one, but the mode of development is 

 identical, there being a forward passage of the strength 

 of the costae and a concomitant freeing of the costae. 



