MOLLUSCA OF MICHIGAN— WALKER. 



457 



I. Genital orifices contiguous or united Monotremata. 



II. Genital orifices separated Ditremata. 



The Ditremata are not represented in the fauna of Michigan. 



The Monotremata are divided into two groups, based mainly on the struc- 

 ture of the lung. Only one of them, the Vasopulmonata, is represented in 

 the fauna of the state. 



The Vasopiilmonata are again subdivided according to modifications of 

 the pallial region, and particularly with reference to the position of the 

 ureter, viz.: 



I. Kidney lying parallel to heart and gut ; the ureter passing direct from 

 the kidney to the anterior margin of the lung Orthurethra. 



II. Kidney forming the posterior wall of the lung, lying transverse to 

 heart and gut, its apex against the latter; ureter following the last fold of 

 the gut forward to the mantle edge Heterurethra. 



III. Kidney parallel to the gut, its apex anterior; ureter abruptly de- 

 flected from the apex, passing to the posterior end of the lung cavity; thence 

 an open groove or closed tube continues across to the last fold of the gut, 

 which it follows f orw^ard to the mantle edge Sigmurethra. 



The Sigmurethra are divided into super-families, based upon characters 

 of the foot, lung, central nervous system, radula, etc., as follows: 



I. Margin of the foot defined by grooves, etc Aulacopoda. 



II. No pedal grooves. 



A. None of the teeth of the aculeate or thorn-like type; jaw present, 

 distinct Holopoda. 



B. All of the teeth aculeate, thorn-shaped; jaw often obsolete. 



a. Cerebral ganglia concentrated, in close contact Agnathomorpha. 



b. Cerebral ganglia separated, a rather long commissure connecting 

 them Agnatha. 



The Agnatha are not represented in Michigan. 



The Heterurethra is composed of the single superfamily, Elasmognatha, 

 characterized by the jaw being furnished with an accessory, quadrate 

 piece, which is lacking in both the Sigmurethra and Orthurethra. 



The subdivisions of the Orthurethra, if any, have not, as yet, been deter- 

 mined. 



Th.e]Holopoda, Agnathomorpha, Aulacopoda, Elasmognatha and Orthurethra 

 represented in the fauna of Michigan, are divided into the famihes shown 

 on the following table: 



CLASSIFICATION OF THE VASOPULMONATA OF MICHIGAN.* 



Holopoda Helicidce. 



Agnathomorpha. Circinariidce. 



SigwMrethra . . . . i f Zonitidce. 



.J 7 I Limacidce. 



Vasopulmonata \' ^ ^ I Endodontidce. 



^ Philomycidce. 

 Heterurethra. . . . Elasmognatha. . . Succiniidce. 



f PupillidoB. 



Orthurethra <| Valloniidce. 



[ Cochlicopidce. 



The Basommatophora, according to Fischer (Man. de Con. 446) are di- 

 vided into three suborders : Gehydrophila, Hygrophila and Thalassophila. 



* Adapted from Pilsbry, Proc. A. N. S. P., 1900, 564. 



