10-1 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



Helices, various morlifications eusued, resulting in the, structurally, 

 most extreme forms of the family; the genitalia, intestinal tract, 

 and shell all being- aiiected. It has been the same with Helicidfe ; 

 the primitive East Asian Belogona of the Eulota type becoming 

 modilicd in the west into the Belogona Siphonadenia, peculiar to 

 the European faunal area. 



That the theories here outlined are the reverse of Dr. Simroth's 

 opinion that the European Arionidse had their centre of distribution 

 in the extreme west of that faunal region, is largely due to the fact 

 that he had not dissected the truly primitive American genera, and 

 naturally drew his conclusions from the highly modified European 

 forms. 



EXPLANATION OF PLATE VII. 



(Figs. 4, 5, aad 6 drawn by E. G. Vanatta ; the others by the author.) 



Fig. 1. Anadenns allivagus (Theob.) : retractor muscles and outline of diaphragm. 



,, 2. The same : pallial region from below, the diaphragm removed. 



,, 3. The same : digestive tract. 



,, 4. Ariolimax columbianus (Gld.) : retractor muscles, from below. 



,, 5. Anadcnus altivagus: genitalia. 



,, 6. The same : penis opened. 



,, 7. The same : teeth. The 53rd tooth is abnormal. 



ao. aorta; G^-'', intestine, first, second, third, and foiu'th folds; k. kidney; 

 L. lung; l.e.r. and l.r. left ocular retractor muscle : l.t.r. left tentacular retractor ; 

 ce. oesophagus; ot. ovotestis ; ph.r. pharyngeal retractor muscle; p.r. retractor of 

 the penis; ret. "retensor" or cephalic retractor muscle; r.o. respiratory orifice; 

 r.r. right tentacular retractor; r.s. radula sack; sp. spermatheca ; v. ventricle; 

 vag. vagina ; v.d. vas deferens ; v.r. vaginal retractors, inserted upon the 

 diaphragm. 



