ss; 
Lo4 GASTRODONTA HEADONENSIS. 
GENUS GASTRODONTA Albers. 
SHELL subperforate or umbilicate, depressed, or orbicularly convex, light horn 
colour, and sometimes transparent ; WHORLS 5-7, finely and obliquely striate ; 
APERTURE lunate, base furnished with longitudinal teeth or folds, which do not 
reach the apertural margin; PERISTOME simple and aeute. 
ANIMAL usually darkly pigmented; FooT long and narrow, with distinct 
locomotor dise ; PEDAL GROOVES distinet, terminating in a longitudinal mucous 
pore; JAW strongly areuate, ends rounded, anterior surface striated, concave 
margin with a well-dey eloped median projection or beak. 
LINGUAL RIBBON bearing a tricuspid central tooth with bienspid laterals and 
aculeate marginals. 
x \ cay 
\ 
Fic. 210. Fic. 211. 
Fic. 210.—Mandible of Gastrodonta [ligera] enlarged (after Bryant Walker). 
Fic. 211.—Representative median, lateral, and marginal teeth of Gastrodonta [ligera] highly 
magnified (after Binney). 
The Gastrodontw are at the present day restricted in a living state to 
North America, and only known in this country by their fossil remains. 
Gastrodonta headonensis (Edwards). 
Helix headonensis Edwds., Mon. Eoc. Moll., 1852, pt. 2, p. 70, pl. xi., f. 5 a-d. 
Gastrodonta headonensis Sandberger, Vorwelt, 1873, p. 276, pl. xiv., ff. 21-24e. 
uA 
Fic. 212.—Gastrodonta headonensis (Edwards), natural size and X 5 (after Edwards). 
SHELL very small and depressly orbicular, with a somewhat elevated SPIRE of 
six or seven rounded WHORLS, separated by : deep SUTURE; APERTURE semi- 
lunar; PERISTOME slightly thickened internally and outwardly reflected, outer 
wall of the last volution bearing three long and parallel longitudinal lamelle 
extending far back, but not reaching the APERTURE; UMBILICUS wide and deep. 
Diam. 2°5 mill. ; alt. 1°25 mill. 
According to Edwards, G. headonensis is extremely 1 rare, but is a well- 
marked and. perfectly distinct species, the specimen figured being merely 
a cast. He remarks on its resemblance to the depressed variety of //. 
labyrinthica, but the greater number of whorls and the different denti- 
tion of the aperture sufficiently distinguish it. 
In general aspect it resembles Vadlonia pulchella, but the spire is more 
elevated, and the whorls are more numerous, while the internal longitudinal 
plaits are especially distinctive. 
Prof. Sandberger remarks on its similarity in some respects to Gastro- 
dontia licmodon,. a native of Alabama and 'Il'ennessee ; but although the 
form and position of the lamellee are quite the same, the recent species is 
without the distinctly margined aperture and is of very much larger size. 
BRITISH ISLES. 
Oligocene—In the Headon limestone at Headon Hill; also in the Bembridge 
series at Hempstead, Bembridge, Whitecliff Bay, etc., Isle of Wight (C. Ashford) ; 
and in the Headon series at Hordwell, Hants, 
