NATUKAL HISTORY SOCIETY. 83 



usually brilliantly so; under 15 long; aperture circular; whorls con- 

 vex; umbilicus large and deep... 69. TroeliidcB. 



U. Shell globular or oval; spire quite short; the body whorl very large ; 

 umbilicus rounded, distinct, either free or covered with a callus ; aperture 



t or more the length of the shell ; angle of spire 90° or more 



67. Natieidw. 



U. Shell usually top-shaped, sometimes globular ; interior not pearly, 8-30 



long; umbilicus if present not rounded nor covered with a callus 



63. Litorinidw. 



U. Shell minute, nearly disk-shaped, widely umbilicated 65. AdeorUdcB. 



V. Large heavy shells, over 100 long, pear-shaped ; whorls angulated or 



nodulous 31. Fasdolariidce. 



V. Rather thin shells, 20-80 long, ovate to pear-shaped; whorls rounded 



and covered with 40-60 small revolving ridges 52. Bueeinidce. 



V. Shells over 20 long, with longitudinal rib-like undulations crossed by 

 revolving lines ; in one species the revolving ridges are more distinct 



than the longitudinal ones , 55. Murieidm. 



V. Shells less than 20 long; aperture with a notch near the posterior end 

 formed by the outer lip not squarely meeting the body wall ; whorls 



shouldered and longitudinally ribbed 50. Pleurotomidce. 



V. Shells less than 15 long; aperture entire at the posterior end, narrow 

 and a little less than half the length of the shell ; canal rather short.. 



54. ColumbelUdce. 



"W. Shell with the apex somewhat spiral, and an internal, usually horizontal, 



partition or diaphragm 66. Cali/ptrceidw . 



W. Shell conical or cup-shaped with apex turned forward ; no shelf nor par- 

 tition. (X.) 



X. No perforation at the apex nor notch in the margin 68. Acmceidce. 



X. Apex recurved ; margin or apex perforated ; shells found either in deep 



water or south of C. Hatteras FissurellidxE . 



Y. The only species in the region, belonging to the order, are in the family... 

 70. Isehnochitonidce. 



