326 Proceedings of the Royal Society of Edinburgh. [Sess. 
especially in those in which the shell is depressed, reduced, or absent, the 
development of the penis rudiment may be postponed until the male 
elements in the gonad are further developed. 
Summary. 
1. An account is given of a fully grown specimen of Helix pomatia, 
which, in addition to the normal set of reproductive organs present on the 
right side, possesses also on the left side a set of accessory male organs — 
namely, a penis and its retractor muscle, vas deferens, and flagellum. There 
is no trace of connection between these supernumerary organs and the 
ovotestis or the genital ducts of the right side. 
2. The normal and supernumerary penes are equally developed and are 
symmetrically placed with regard to the median plane of the head. 
3. The extra penis has the usual structure ; it possesses a fully-developed 
copulatory organ and a muscular tubular introvert upon the inner sheath. 
4. The extra vas deferens opens at one end into the penis and at the 
other terminates blindly in the sub-epidermal tissue ; the blind end is bound 
to the epidermis by a short cord of connective tissue. 
5. The extra flagellum is normal in size and in its internal structure. 
6. The penial nerves on both sides have a corresponding course from 
the cerebral ganglion to their distribution on the penes. Owing to defective 
preservation it was impossible to determine whether the fibres of the extra 
penial nerve arise from the ocular nerve, as is apparently the case, or 
whether they really arise from the pedal ganglion, as do those of the normal 
penial nerve. 
7. The penis opens into a short atrium which communicates with the 
exterior by a genital aperture of normal size and shape. This super- 
numerary genital aperture occupies a position on the left side exactly 
corresponding to that of the normal one on the right side. About 2 '5 mm. 
posterior and slightly dorsal to it is the blind end of the vas deferens, which 
is situated close to the dorsal margin of the epidermal groove, which tra- 
verses the anterior portion of the animal. 
8. In the duplication of the penis and its associated structures this snail 
presents an example of lateral homoeosis. The only comparable cases 
recorded in the Mollusca are a Pteropod with paired penes (see Addendum, 
p. 327) and a specimen of Moschites cirrosa, in which not only was the third 
right arm hectocotylised as usual, but the third left one also. 
9. The supplementary vas deferens and the retracted penis form a 
U-shaped loop, both ends of which are in contact with the epidermis, and 
