MONUGUAl'II OF lUUTISlI LAND ANli FRESH-WATKU MOLLUSCA. 



'20 1 



(lENUs GEOMALACIJS Alliiian. 



Ceomalacus, Alliiian, Allnjn.ijiiiii, 1,SJ2, p. S51), 



f(A. XAilAA^ 



dA'^' 



HE genus Geomalacus (yij, the earth; 

 /;.uA.a)(05, molhisk) is dedicated to the 

 late Prof. G. J. Alhnaii, who instituted 

 tlie group and clearly detined the dis- 

 tinguisliing features of its external 

 niorjihology. 



The genus has heen siihdivided by 

 continental autliors, and the sulj- 

 genera Lrtdunirdii.r/u and Amuliji: 

 established fur the sjiecies inhabiting 

 Xiirtli-western Africa and the southern 

 ])arts of Spain and Portugal, which are 

 distinguished by their internal organi- 

 zation approximating more closely t(j 

 that of Arioii, and externally by 

 possessing distinct and dark lateral 

 banding in adult life, a. feature espe- 

 cially characteristic (jf and restricted 

 to early life in the animals of the 

 typical genus. 



This dermal pigmentation is be- 

 lieved to ])romote cutaneous respira- 

 tion,' and in its ]iosition to lie 

 essentially ilejiendcut u]ion ,'iud to 

 indicate tlie course of the blood. 



Generic Characteristics. — Kxtkkxallv, (Ai^oukiIkcux has a sub- 

 cyliudrically lima(nfirm and non-carinate 1!oj>y, 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 ukspiratouv 

 ORIFICE placed on the right margin of the shield and near the anterior 

 third of its length ; in the young, however, the respiratory orihce is 

 ipiite median or even behind tlie centre, but as the growth of the shiehl 

 jiroceeds more ([uiekly behind than in front, the aperture becomes anterior 

 by the time full growth has been attained ; genital orifice near to and 

 below the right ommato])hore; caudal oland opening transversely between 

 the foot and the boily : F(jot-sole indistinctly trifasciate, the median- 

 area slightly darker and more tra,nsiia.rent than the sides, and showing 

 the muscular waves of extension and c(uitraction during locomotiou. 



The shell is represented b}- an oval, flat and solid calcareous jilate, 

 placed beneath the shield a)id abo\e the respiratory chanifjer. 



Internally, the various species show a remarkable modification in the 

 an-angement of the reproductive system, which exhiliits an extraordinarily 

 lu'(jlonged ATRii'M or vcstiliule, which fuuctious as a mali^ organ, \vhile 

 its retractor mirscle is, according to the I'csearches of I'ilsbry, a totally 

 distinct structure from that of .!/■/(;//, :ui(l is aflixed at the re;ir of the 



1 M u. i,, li, :i"i;. 



