THE NATURAL HISTORY SURVEY. 169 
Remarks: The variety (which was formerly known as typ- 
ical monodon) may be distinguished from the typical form by 
its larger shell, more hirsute surface and generally less widely 
open umbilicus. It is very abundant in the Big Woods west of 
Evanston, where it attains a large size and great perfection. 
It is rather widely distributed but has not yet been found in 
the southern region. The forms having an imperforate um- 
bilicus are rare and have only been found in the western re- 
gion. 
Superfamily Agnatha. 
Mouth without jaw; radula with numerous sharp, aculeate 
or fang-shaped teeth, the center tooth wanting or rudimen- 
tary; neck generally elongated and furrowed along the back; 
cerebral ganglia separated, connected by arather long com- 
missure. 
FAMILY SESTACHKELID:. 
“Animal slug-like, bearing a small ear-shaped shell near 
the posterior extremity of the body. Nojaw. Lingual teeth 
_long and narrow, sharp-pointed, in oblique series.”* 
Genus TESTACELLA Cuvier. 
“Animal: Limaciform, subcylindrical, tapering anteri- 
orly; tentacles simple; mantle small, posterior, quite near the 
tail, covered with a small external shell; no longitudinal fur- 
rows above the margin of the foot, and no caudal mucus pore; 
no distinct locomotive disk; external respiratory and anal ori- 
fice at the posterior right edge of the mantle under the peris- 
tome of the shell; combined generative orifice behind and be- 
low the right eye-peduncle.” 
“Shell; External, rudimentary, imperforate, ear-shaped, 
with a subspiral, posterior nucleus.” (Tryon.) 
Distribution: Europe, Canary Islands, United States (in- 
troduced). 
63. Testacella haliotoidea Fer., pl. xxxi, fig. 24. 
Testacella haliotoidea FER., Hist. Nat. Des. Moll., pl. viii, Figs. 5-9, 
1820. 
Shell: Small, ear-shaped, partly spiral; apex small, sub- 
spiral, posterior; columella broad, flat, excavated; surface 
*Tryon,S. and S. Conch., Vol. 3, p. 11. 
