346 A MANUAL OF AMERICAN LAND SHELLS. 



Jaw abseut. Lingual membrane narrow, with chevron-shaped rows 

 of uuiform, aculeate, separated teeth ; central tooth with a long, slender, 

 straight base of attachment, with incurved sides and with inferior, 

 lateral, sliglitly expanded angles, and with the upper margin reflected 

 and extended into a long, slender, acutely pointed cusp. There are no 

 lateral teeth, the balance of the membrane being composed of marginal 

 teeth of the pure aculeate form. Each row of teeth on either side of 

 the median line curves first backward, with the teeth rapidly increas- 

 ing in size as they pass outwards, and then forwards as the teeth grad- 

 ually again become smaller, giving an irregularly cresceutic shape to 

 the half-row of teeth. This is shown particularly in Gl. Albersi and 

 G. rosettj less so in GI. truncata. The central tooth was overlooked by 

 Wyman, Lei«ly, and other of the earlier investigators. It has since 

 been detected in GI. truncata, rosea, algira, Sowerbyana, pUcatula, fusi- 

 form is, Albersi ; in semitarum, PhilHpsi, of the subgenus Varicella; also 

 ,solidula, of subgenus Oleacina. This central tooth is rather difficult to 

 istudy, being on a different plane from the other teeth and apparently 

 much less developed. Its cusp is generally simple, long, and narrow; 

 but in G. rosea it has a decided blunt cutting point, and in G. semitarum 

 it has a long, slender cutting point; for that of G. truncata see be- 

 low. 



The side teeth are all of the purely aculeate type ; the base of attach- 

 ment is long, narrow, incurved at sides, gradually rounded above, ex- 

 panded and bluntly truncated below, the general outline being some- 

 what like that of the sole of a shoe. From this base of attachment 

 S!)rings a large, aculeate cutting point. These side teeth are like the 

 marginals in Zonites, Limax, &c. ; they may therefore be called mar- 

 ginal teeth, and the lateral teeth, usually present intho, Liniacidce, may 

 be said to be entirely wanting. 



As stated above, the marginal teeth increase rapidly in size for a 



^ „,, short distance from the median line, 



Fig. 375. ' 



and then gradually decrease in size 

 as they pass off laterally, the last 

 tooth being still smaller than the 

 first. 

 ''''■ In illustrating the dentition of this 



Lingual dentition ot G. truncata gCUUS I refer tO the figure OU p. 345, 



to show the general arrangement en chevron of the rows of teeth. The 



