NATURAL SCIENCES OF PHILADELPHIA. 161 



cusp. The other figures are drawn from below, to show the base 

 of attachment. 



31. Vancouverensis^ drawn by Morse, has 22 — 1 — 22 teeth, two 

 other membranes examined by me gave 24 — 1 — 24, one other 

 18—1—18. M. concava has given 20—1—20, 23—1—23, and 

 25 — 1 — 25. Of 31. Duranii I have counted but one membrane 

 having 18 — 1 — 18. A single membrane of 31. Voyana had 24 — 

 1—24 teeth. 



To sum up the characters of the dentition of 3Iacrocyclis it 

 may be said to be intermediate between Glandina and Zonites, 

 differing from the former in the presence of the transition teeth 

 from true laterals to true marginals, differing, however, from tlie 

 latter by the absence of true lateral teeth. 



Genus ZONITES, Montf. 



In the preceding genus Glandina we found only the aculeate 

 form of teeth or pure marginals ; in 31ac7'Ocycliii we found in addi- 

 tion to these marginals a few showing a modification of this type, 

 being the transition teeth from marginals into laterals. In the 

 present genus, Zonites, we find for the first time the latei'al teeth 

 in their full development. Thus we have the three forms of teeth, 

 centrals, laterals, and marginals,. all present, and apparent!}' a 

 generic characteristic. It will be noticed, however, that in laevi- 



Fi.-. 5. 



General view of dentition of Zonites indentatus. 



gatus^ (pi. II., fig. 1, 2) there is no perfect lateral, the first tooth 

 showing a decided modification or transition into the marginals. 

 Thus we cannot say that in all species of Zonites there are pure 

 lateral teeth. It will be seen below that in some species the num- 

 ber of laterals is reduced to two. 



' See also cellarius. 



