286 DE. H. J. FLEUEE ON THE EVOLUTION OF 



I tliink he means, that hoth groups have descended from a common ancestor nearer the 

 Monobranchs, Tliiele's difficulty ahout the ctenidial vestiges disappears. It is easy to 

 understand that along one line of development both ctenidia were early reduced, while 

 along the other the reduction of one was arrested, so that it persisted in a proh;il)ly 

 sliglitly degenerate state. The respiratory importance of the branchial roof in Patellidae 

 explains the maintenance of the osphradial sensory structures. 



Tliis view of the affinities of the subgroups, whicli is, I think, in full accord with 

 Pelseneer's opinion, is also supported by the presence of probable vestiges of the 

 eijipodiu'Ji among the Cyclobranchs ; the common ancestor would have retained this 

 ancestral organ to some extent. The odontophore also shows that the two orders have 

 diverged along somewhat difTerent lines. The account given here of the consolidation of 

 the viscera shows, I think, that the Monobranchs are the more primitive group, but 

 that the ancestor of both may be supposed to have been intermediate between Acmcea 

 galalhea, for example, and Ancistromesiis. 



The affinities of the Docoglossa with other forms are difficult to trace on account 

 of the antiquity of the group. The Haliotidse, Fleurotomaria, and the Trochidse have 

 evolved far along other lines, especially as regards the shell-musele, the branchial cavity, 

 the visceral loop of the nervous system, the disposition of the visceral hump and 

 its contents, the heart and pericardium, and the kidneys. Scissurella is certainly some- 

 what more like the Docoglossa in external features in some species and in the condition 

 of the branchial cavity, but it is in other respects nearer the Haliotidse &c. The 

 Fissurellidae have the same form of the shell-muscle and the condition of the kidneys is 

 similar to that among the Docoglossa, but it is probable that among them the complete 

 external symmetry is secondary. They have, besides, evolved on lines of reduction of 

 the shell and unique sjjecialization of the branchial cavity, while the process of consoli- 

 dation of the viscera was also probably very different. It is possible that the Docoglossa 

 may be connected with the Bellerophontacea, which possessed symmetric shells that in 

 some cases show a tendency towards expansion of the rim, analogous to that which has 

 had such far-reaching effects among the Docoglossa. This is, however, quite problematic, 

 owing to our very limited knowledge of the palaeozoic Gastropods. We are only 

 justified in hinting that the Docoglossa and Bellerophontacea are two of the earliest 

 offshoots from the Gastropod stem. 



A List of Papees Dealing with the Docoglossa. 



T. Adams, L. E.— Deep Limpet " Scars" [of Patella vulgata']. 'Naturalist,' 1890, p. 335. 



2. Amaudrut, a. — La partie aiiterieure du tube digestif et la torsion cbez les mollusques gasteropodes. 



Ann. Sci. Nat., Zool. s^r. 8, t. vii., 1898. 



3. Bernard, F. — Recherclies sur les organes palleaux des Gasteropodes prosobranches. Ann. Sci. Nat., 



Zool. ser. 7, t. ix., 1890. 



4. Boutan, L. — L'organe glandulaire periph(irique de l'i/e/«o«/ie//Md6?Mm. Arch, de Zool. exp. s6r. 3, 



t. v., 1897. 



5. BoTJTAN, L. — La cause principale de I'asymetrie des Gasteropodes prosobranches. Arch, de Zool. 



exp. s6r. 3, t. vii., 1899. (Includes an account of development of Acmcea.) 



