MONOGRAPH OF BRITISH LAND AND FRESHWATER MOLLUSCA. 251 
GENUS GEOMALACUS Allman. 
(Geomalacus, Allman, Athenwum, 1842, p. 851). 
HE genus Geomalacus (yy, the earth ; 
paXdayos, mollusk) is dedicated to the 
late Prof. G. J. Allman, who instituted 
the group and clearly defined the dis- 
tinguishing features of its external 
morphology. 
The genus has been subdivided by 
continental authors, and the sub- 
genera Letourneauria and Arrudia 
established for the species inhabiting 
North-western Africa and the southern 
parts of Spain and Portugal, which are 
distinguished by their internal organi- 
zation approximating more closely to 
that of Avion, and externally by 
possessing distinct and dark lateral 
banding in adult life, a feature espe- 
cially characteristic of and restricted 
to early life in the animals of the 
typical genus. 
This dermal pigmentation is be- 
heved to promote cutaneous respira- 
tion,' and in its position to be 
s ‘ UA essentially dependent upon and_ to 
indicate the course of the blood. 
Generic Characteristics. — ExrerNaLiy, Geomalacus has a sub- 
cylindrically lmaciform and non-carinate Bopy, with a bluntly-rounded 
caudal extremity ; MANTLE or shield granulose, about one-third of the 
total length of the animal when extended, with the ample RESPrRATORY 
ORIFICE placed on the nght margin of the shield and near the anterior 
third of its length; im the young, however, the respiratory orifice is 
quite median or even behind the centre, but as the growth of the shield 
proceeds more quickly behind than in front, the aperture becomes anterior 
by the time full growth has been attained ; GENITAL ORIFICE near to and 
below the right ommatophore; CAUDAL GLAND opening transversely between 
the foot and the body; Foot-soLr indistinctly trifasciate, the median- 
area slightly darker and more transparent than the sides, and showing 
the muscular waves of extension and contraction during locomotion. 
The SHELL is represented by an oval, flat and solid calcareous plate, 
placed beneath the shield and above the respiratory chamber. 
INTERNALLY, the various species show a remarkable modification in the 
arrangement of the reproductive system, which exhibits an extraordinarily 
prolonged arrium or vestibule, which functions as a male organ, while 
its retractor muscle is, according to the researches of Pilsbry, a totally 
distinct structure from that of Avvon, and is affixed at the rear of the 
1 Monog. i., p. 306. 
