Vlll AGNATHA. 



SUPERFAMILY AGNATHA. 



The AGNATHA have a distinct pedal groove, defining narrow 

 foot-margins as in the Aulacopoda, and two lateral grooves 

 diverge from the mantle forward. The teeth are all thorn- 

 shaped, with single simple or barbed cusps. By secondary 

 modification the shell in existing forms is much reduced or 

 absent, and consequently no part of the viscera occupy a 

 spiral protuberance. They are contained in the cavity of the 

 foot which is excavated into the tail. The cerebral ganglia 

 are generally united by a rather long commissure. The re- 

 tractor muscle system is more or less modified, as is usual 

 when the shell is reduced. The genital system is simple. 

 The jaw is well developed in Plutonia, very weak and hard 

 to distinguish in the Caucasian genera, and merely vestigial 

 in Testacclla. 



The relationships of the Agnatha are with the Aulacopoda, 

 not with the Agnathomorpha. The following families are 

 distinguishable. 



Family TESTACELLID.E Gray. 



Mantle very small, posterior; shell external, Haliotoid or 

 Vitrinoid, much too small to contain the soft parts. No dis- 

 tinct jaw. Palsearctic. 



Testacellinae : genus Testacella. 



Daudebardiina?, genus Daudebardia. 



Family TRIGONOCHLAMYDID/E. 



Mantle small or moderate, submedian, with posterior pneu- 

 mostome, the tail behind it keeled. Shell internal, vestigial. 

 Jaw more or less developed. 



Trigonochlamydinge. Caucasian genera with the jaw very 

 weakly developed, shell minute or wanting, mantle marked 

 with a curved groove. Palsearctic, Caucasus. Genera Tri- 

 gonoclilamys, Phryxolestes, Hyrcanolestes, Pseudomilax and 

 perhaps Selenochlamys. 



Plutonimas. Azores Islands; jaw well developed, smooth; 

 shell Ancylif orm ; no curved groove on the mantle. Genus 

 Plutonia. 



