282 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



{Gymnarion) gomesianus (Morelet)/ and in H. {Granularion) 

 cryptophallus, Watson.^ 



It is thus evident that a sub-cerebral commissure has now been 

 found in at least six families of the Stylommatopbora alone, namely, 

 the Zonitidae, the Helicidse, the Acavidse (subfamily Strophocheilinae), 

 the Achatinidae, the Rhytididse, and the Aperidse. But it will be 

 observed that all these families belong to the Sigmurethra. So far 

 as I am aware, a sub-cerebral commissure has never been stated to 

 occur in any member of the Orthurethra. This is remarkable, not 

 only because of the large number of genera that belong to this division 

 of the Stylommatophora, but also because the Orthurethra is 

 supposed to be a rather more primitive group than the Sigmurethra, 

 mainly on account of its excretory organs being more like the type 

 found in the Basommatophora. It might, therefore, be inferred 

 either that the Orthurethra is not really more primitive than the 

 Sigmurethra, or that the sub-cerebral commissure found in the 

 higher Pulmonates should not perhaps be regarded as a primitive 

 structure, but might possibly have arisen by the anastomosis of a 

 pair of cerebral nerves. 



In view of these facts, it seems well to make known that I have 

 lately discovered that a sub-cerebral commissure is present in at 

 least three different genera belonging to the Orthurethra. I have 

 found it in full-grown specimens of Ena ohscura (MiilL), from the 

 Gog Magog Hills near Cambridge, in an immature example of Rachis 

 punctata (Anton), from Bombay, kindly sent to me by Col. Peile, 

 and in a full-grown specimen of Chondrina similis (Brug.), 

 from Alassio on the Italian Riviera, for which I am indebted to 

 Major Connolly. That all these species are correctly assigned 

 to the Orthurethra I have proved by an examination of their 

 excretory organs. 



In these species the sub-cerebral commissure does not exceed 

 •005 mm. in diameter, apart from the surrounding connective tissue ; 

 this is scarcely one-tenth of the diameter of the cerebral commissure, 

 which, however, is not nearly so long. As in other Pulmonates, it 

 arises from the outer and lower side of each cerebral ganglion, a 

 short distance in front of the origin of the cerebro-pedal connective, 

 and close to the origin of the cerebro-buccal connective. It passes 

 round the oesophagus anterior to the cerebro-pedal and cerebro- 

 pleural connectives, and in Ena ohscura and Chondrina similis — and 

 possibly also in Rachis punctata — it is attached for the greater part 

 of its length to the front of the two lateral cephalic arteries. In the 

 centre, where these arteries arise from the anterior aorta, the com- 

 missure passes straight across, underneath the lower end of the 

 odontophoral or buccal artery, but in front of the origin of the pedal 



1 Proc. Malac. Soc, vol. xiv, 1920, p. 94. 



2 Ibid., p. 99. 



