MORPHOLOGY OF JAW. XI 



ing toward the middle below, the median plate or plates triangular,, 

 not reaching the cutting margin (Plectopylis*). 



Aulacognath (or striated, pi. 15, figs. 1, 2) primary elements or 

 plates completely soldered together, vertically striated (Pyramidula)- 



Oxygnath (or smooth, pi. 21, fig. 8) completely soldered, smooth 

 (Leucochroa). 



Odontoguath (or ribbed, pi. 21, fig. 11) completely soldered, hav- 

 ing convex vertical ribs, projecting at one or both edges (Helix). 



The most primitive type of jaw occurring in recent terrestrial 

 Pulmonata is found in the Polyplacognatha, Punctum and Laoma. 

 By the partial union of the loose plates of this sort of jaw, the 

 Stegognathous type is formed. The goniognath form as seen in 

 Licjuus, Orthalicm, etc., is a mere variant of this low stegognath 

 type, and can hardly be considered a primary type. In the Aulaco- 

 gnatha the plates have become completely soldered, although their 

 edges still show as stripe ; and finally in the Oxygnatha these strife 

 disappear, leaving a completely smooth jaw. In the Odontognatha, 

 vertical ribs are developed upon its anterior face. The data supplied 

 by anatomy and embryology indicate the above as the general phy- 

 logenetic sequence of the various types of jaw ; but the Oxygnatha 

 consist of two sections of different genesis. In some forms (such as 

 the typical Sagdas) the jaw has apparently been evolved directly 

 from the stegognathous type ; and this is probably true likewise of 

 the Helicophanta group. In others (such as some species of Pleu- 

 rodonte, and Helicostyla,t\\e genera Obba, Cepolis, Leucochroa, Allo- 

 gnathus, etc.) a smooth jaw has resulted from the degeneration of 

 the ribs on an odontognathoustype. The ribbed orodoutognathous 

 type has in some cases been formed upon a plaited jaw. In other 

 cases it may have been formed upon a smooth jaw, but evidence is 

 lacking to establish this. In certain cases (such as Hygromia') the 

 degeneration of a ribbed jaw has resulted in one approaching the 

 plaited type. It must also be understood that the distinction 

 between the goniognathous, stegognathous, aulacognathous and 

 oxygnathous types is in some cases not well defined, and often it is 

 not possible to distinguish between a primarily or secondarily oxy- 

 gnathous or smooth jaw, although it is practically demonstrated that 

 the Oxygnatha are diphyletic. 



It therefore appears that at the time the main phyla of monotre- 

 mate, jaw bearing land snails diverged, they were provided partly 

 with a jaw of unsoldered plates, partly with one of the incompletely 



