SHAW : ON CYPR^A AND TRIVIA. 291 



very kindly undertook to work out the anatomy of specimens of 

 Cyprc^a tigris and Trivia arctica. 



The difference between Cyprcea and Trivia lies first of all in the 

 nervous system of the foot. 



In Cyprcea, as originally described by Bouvier/ the pedal centres 

 are in the form of a long pair of cords (PI. XII, Fig. 3), swollen at 

 their anterior extremities and composed of a central core of nerve 

 fibres, ensheathed by nerve ganglion, cells throughout their extent. 

 These two longitudinal pedal cords are connected by a number of 

 transverse commissures, of which the most anterior, connecting the 

 swollen anterior extremities, is the largest and most important. This 

 scalariform system of transverse commissures is, as Bouvier points out, 

 a primitive feature, recalling the condition found in Patella and 

 Paludina [ Vivijjara^. 



In Trivia the pedal centres are much concentrated when compared 

 with those of Cyprcea. Whereas in the latter the length of the pedal 

 centres relatively to that of the foot is as 3 to 4, in Trivia it is as 

 1 to 14. 



The part corresponding to the swollen anterior ends of the cords in 

 Cyprcea, with the anterior transverse commissure connecting them, 

 remains ; but the posterior elongated cords are very much abbi"eviated, 

 and, indeed, are so small as to be only recognizable in sections. In 

 dissection they loolc like a stout pair of nerves given off from the 

 posterior ends of a pair of rounded pedal ganglia. Sections (PI. XIII, 

 Figs. 1-7), however, show that these apparent nerves are, like the 

 pedal cords of Cyprcea, ensheathed by ganglion cells to their hinder- 

 most ends, and that the nerves supplying the foot are given off from 

 their sides. There are, however, no transverse commissures beyond 

 the one already mentioned, and therefore no trace of the scalariform 

 system observed in Cyprcea. Trivia therefore is more specialized in 

 the nervous system than Cyjjrc^a. 



In the second place, the radula^ are distinct; in that of Cyprcea 

 tigris (Fig. 1) the median tooth has a large central cusp with a small 

 one on each side ; the marginal teeth have an elongated and hook- 

 shaped central cusp with a small one of similar shape at its base, but 



they are not much longer in proportion than the median, and the ends 

 of the marginal teeth do not extend so as to meet in the middle of 

 each transverse row of the radula ribbon. The lateral teeth have one 

 central cusp and a small pointed one on each side. 



1 Bibl. de I'ecole des Hautes Etudes, 1887, vol. xxxv, p. 216. 



