1904.] 



NATURAL SCIENCES OF PHILADELPHIA. 



823 



very small, species, few-whorled, with a clausilium very deeply excised 

 on the columellar side of the filament, and abruptly bent and twisted 

 there. The closing apparatus is always lateral or dorso-lateral, the 

 lamellae rarely penetrate as far as the middle of the ventral side, and 

 the spiral lamella is usually low. 



The sections Oligozaptyx and Diceratoptyx constitute two new second- 

 ary phyla, independently evolved from the typical group. In both 

 the clausilium is notched, and the whole structure shows great speciali- 

 zation. Another line of differentiation is represented by the sections 

 Metazaptyx and Parazaptyx, in which the inferior lamella has become 

 spiral, thereby obstructing the throat, and the clausilium is strongly 

 curved. In Parazaptyx the clausilium is further modified in a peculiar 

 and unexampled manner. Then we come to a series of groups in which 

 the shell is without accessory plicae and lamellae, and now defined as 

 Hemizaptyx and Stereozaptyx. The species of these groups I had 

 formerly appended to various sectional divisions as aberrant forms. 

 The evidence now at hand indicates that they are either degenerate 

 or primitive branches of the Zaptyx phylum. Stereozaptyx may 

 well be a secondarily simplified form of Parazaptyx, in which the 

 accessory plicae and lamellae have been obliterated in the general and 

 extreme thickening of the shell walls. If so, immature shells might 

 possibly still show some trace of them. 



The interrelations of the subordinate groups and their approximate 

 phylogeny, so far as I understand the recent and fossil forms now 

 known, may be represented by the following diagram. The names of 

 those groups of which the position is hypothetical are in italics. 



Oligozaptyx Diceratoptyx 



Zaptyx 



Parazaptyx 



Metazaptyx 



Stereozaptyx 



Hemizaptyx 



