354 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



than wide, with incurved lower margin and expanded lower lateral angles ; 

 the upper margin broadly reflected ; reflection short, stout, with suboljisolete 

 side cusps bearing no cutting points, and a stout, long median cusp bearing a 

 short, blunt cutting point, which does not reach the lower margin of the 

 base of attachment ; the reflection with the median cusp is pear-shaped ; in 

 many species there is a duplicate line of reinforcement parallel to the upper 

 margin of the base of attachment. The lateral teeth are of similar type to the 

 centrals, but are asymmetrical by the suppression of the inner, lower, lateral 

 angle of the base of attachment. The outer laterals have a side cusp and cut- 

 ting point. The transition from laterals to marginals is formed by the greater 

 proportional developuicnt of the cutting point, the lesser development of the 

 cusp ; the cutting point then becomes bifid, the reflection becomes more nearly 

 the same size as the base of attachment, and thus the true marjrinals are grad- 

 ually reached. Tliese last are longer than wide, have a base of attachment 

 smaller than the reflection and cut away on its lower inner angle ; the reflec- 

 tion is produced into one long, sharp, oblique, bifid cutting point, the inner 

 division the smaller, and one outer, much shorter, sharp, rarely bifid cutting 

 point. 



Most of the species examined agree in dentition with Stearnsiana. Some 

 have more blunt cutting points to their marginals, as sequoicola (PI. IX. Fig. J), 

 but even on various parts of the same membrane the marginals vary in this 

 respect. In Kelletti, Stearnsiana, tudicutata, arrosa, Traski, sequoicola, Ayresi- 

 ana, redimita, Nickliniana, ramentosa, exarata, Diahloensis, facta, Carpenteri, I 

 have failed to detect any side cutting points to the central and inner lateral 

 teeth. I found the points, however, in A. nijicincta (PI. IX. Fig. N). A. 

 Townsendiana (PI. IX. Fig. Q) has these cutting points and side cusps on cen- 

 tral and all the lateral teeth ; its centrals and laterals are not of the same shape 

 as described above, but resemble those of Polygyra, Stenotrema, and Triodopsis. 

 Thus in this as in other genera we find the type of dentition not constant 

 in all the species. 



The long, narrow base of attachment and pyriform reflection of most of the 

 species of Arionta agree with those of Hemitrochus more nearly than any other 

 of our genera, but that genus has quite different marginal teeth. 



The dentition of A. arhustorum is alone known of the species foreign to 

 America, and that by a figure of Lehmann (Lebenden Schnecken, PI. XI. Fig. 

 29) too unsatisfactory to be of value for the purpose of comparison. 



Arionta arrosa, Gould. 



Shell globose-conic, thick, umbilicated, indented, and minutely granulated ; 

 color reddish-olive, varied with yellow, and with a fuscous revolving band ; 

 whorls 7, convex ; aperture roundly ovate ; peristome reflected, flesh-colored ; 

 throat bluish. Diameter, 40 mill.; height, 18 mill. 



