Xll MORPHOLOGY OF JAW. 



united type (stegognathous or plaited). In the Helicoids the major- 

 ity of forms acquired the firmer and completely united smooth or 

 ribbed type, although some still retain the primitive, incompletely 

 united forms, as seen in Punetum, Flammulina, Thysanophora, etc. 

 In the Zonitidce the oxygnathous type has been very generally 

 acquired, although a few forms retain a modified plaited jaw. In 

 JBulimulidce (which includes the " Orthalicidae ") the plaited type of 

 jaw has been retained with various modifications, and the same is 

 found in Cylindrellidte. The Papidce have a completely united, 

 striated jaw. The Achatinidce have a striated or ribbed jaw. It 

 appears that the various families, starting with an incompletely 

 united jaw, have been very unlike in the degree of development 

 attained ; some preserving the ancestral form until to-day, but in 

 most a stronger, solid jaw has been acquired through various well 

 understood successive stages, occasionally parallel in several phyla. 

 These considerations show that the various classifications of land 

 mollusks by jaw characters are artificial ; the various " types " of 

 jaw on which it is founded representing merely successive stages of 

 progress from an incoherent or incompletely united, to a solid jaw, 

 and these stages have been independently reached or passed through 

 by several totally diverse branches of the pulmonate trunk. The 

 ihistory of the various jaw types is shown in the following diagram- 

 ribbed smooth 



striated 



plaited goniognath 



Jaw of distinct plates 



The two lower stages were probably passed through by the majority 



<of families in common ; the others were reached by various groups 



independently and by their own special routes. In most families of 



land snails, two or more of these types are represented among the 



various genera. 



THE RADULA in Helicidce is of the strap-like form usual in Pul- 

 monata, the individual teeth having squarish basal plates. In even 

 the lowest types now existing, the multicuspid form of tooth of the 

 primitive Pulmonates has given way to the tricuspid type (see pi. 



