WATSON : AFFINITIES OF PYRAMIDULA, ETC. 9 



The researches of Simroth,^ Pilsbry,^ and others have shown that 

 the Stylommatophora may be divided according to the characters 

 of the excretory system into four main groups, the Sigmurethra, 

 the Orthurethra, the Heterurethra, and the Clasturethra, the great 

 majority of the families belonging to the first two of these groups. 

 In the Sigmurethra, to which both the Endodontidse and the 

 Helicidse belong, the ureter arises from the front end of the kidney, 

 runs back along its upper edge, and then bends round at the hind 

 end of the mantle-cavity and passes forward immediately beneath 

 the rectum, until it reaches the region of the respiratory opening. 

 The first part, running backwards beside the kidney, is generally 

 known as the primary ureter ; and the second part, that runs beside 

 the rectum, as the secondary ureter. In a few of the most primitive 

 members of the Sigmurethra the ureter throughout its length 

 merely consists of a shallow open groove; and in many other genera, 

 while the primary ureter takes the form of a closed duct, the 

 secondary ureter remains open. But in most of the more highly 

 organized snails both parts of the ureter are closed throughout, 

 and this is the case in Goniodiscus rotundatus, as will be seen from 

 text-fig. 3c. This species is clearly a typical member of the 

 Sigmurethra. 



In the Orthurethra, a group which includes the Pupillidse, 

 Cochlicopidse, and Enidse, the ureter follows a quite different course, 

 for it passes straight forward from the front end of the kidney, 

 parallel to the rectum but some distance below it. Just before 

 reaching the mantle-edge the ureter terminates, its end being 

 slightly bent upwards, and the opening being on the dorsal side 

 of the extremity. From this point there runs backwards, along the 

 upper side of the ureter, a shallow groove in the roof of the mantle- 

 cavity, lined by an epithelium similar in character to that which 

 lines the ureter itself. Now this description ajDplies in every detail 

 to the excretory system of Pyramidula rupestris, Patulastra balmei, 

 Acanihinula lamellata, and the three species of Vallonia ; that is 

 to say, these forms all belong to the Orthurethra. In Vallonia the 

 kidney and ureter are relatively shorter than in Patulastra or 

 Pyramidida, as will be seen on comparing text-figs. 3a and 3h;^ but 

 this is not an important difierence, and is probably due to the 

 whorls being .fewer in number and the mantle-cavity shorter in 

 consequence. 



In Acanthinula aculeata we find a very interesting modification 

 of the orthurethrous type. In this species the groove that runs 

 backwards along the upper side of the ureter from its anterior 

 opening has been converted into a closed duct ; and the actual 



^ Semper's Reis. in Arch. Philip., iii, 1894, p. 70 ; Bronn's Tier -Reich, 

 vol. iii, 1911, pp. 374-437. 



2 Proc. Acad. Nat. Sci. Phila., 1900, p. 561 ; Man. Conch. (2nd ser.), 

 vol. XX, 1910, p. vii. 



'^ See also pi. I, fig. 1, and pi. II, fig. 3. 



