24 GILBERT C. BOURNE. 



The cerebral commissure is long and situated far forward 

 in front oF the anterior pair of salivary glands. It is a true 

 nerve, not ensheathed by a layer of ganglion cells, differing 

 in this from Pleurotomaria. The cerebral ganglia are of large 

 size, sub-triangular in transverse section, and produced into 

 prominent lobes at the origins of the more important nerves. 

 The tentacular and optic nerves have separate origins from 

 the cerebral ganglia, Incisura agreeing in this point with 

 Trochus and Fissurella but differing from Pleurotomaria. 

 The labial lobe is very large, and forms a long, conical, taper- 

 ing, antero-ventral process of the cerebral ganglion, which 

 curves inward below the odontophore on either side, maintain- 

 ing its thickness for about two thirds of its course towards 

 the middle line. Then it tapers abruptly to form a thin labial 

 nerve, which passes between the muscles of the lower lip, and 

 as far as I can determine is connected by an extremely fine 

 prolongation with its fellow of the opposite side, thus com- 

 pleting the labial commissure. The buccal commissure is 

 given off from the labial lobe about half way between the 

 cerebral ganglion and the mid-ventral line. It passes inwards 

 among the muscles of the odontophore and at once turns 

 abruptly upwards to run between the extrinsic and intrinsic 

 muscles to the top of the buccal bulb. Here it enlarges to 

 form a ganglion of considerable size, lying close to the inside 

 of the cerebral ganglion, and from this a stout nerve — a true 

 nerve without a sheath of ganglion cells — passes inwards and 

 backwards over the top of the odontophore and enlarges below 

 the origin of the oesophagus into a small ganglion, which is 

 connected by a very short commissure with its closely adjacent 

 fellow of the opposite side. Bouvier (3) has figured and 

 described two swellings at the ends of each of the elongated 

 buccal ganglia of Turbo setosus, but I infer from his descrip- 

 tion that the}' are not separate ganglia, but merely swellings at 

 the ends of a long and ill-defined ganglion. I find precisely 

 the same arrangement in Fissurella grteca, but Boutan 

 figures four clearly defined ganglia in F. reticulata. The 

 sub-division of this elongated ganglion into two distinct 



