TOPOGEAPHICAL EELATIONS AMONG THE DOCOGLOSSA. 275 



Part II. — The Docoglossa. Foot and Edge of Shell. 



The present attempt to sketch the evolution of topographical relations among the 

 Docoglossa is based upon a detailed study of Acmcea virginea, A. teshidinalis, A. corti- 

 cata, and A. fragiUs, Patina pelluclda, a species of Nacella, and Patella vulgata, 

 P. ccerulea, and P. ornuta. I have also had the advantage of Haller's notes on Acmcea 

 galathea (he uses the name Scutellina galathea), Lottia viridula, Scurria, Nacella, 

 Anc'istromesus, and Patella magellanica. The chief types of structure within the 

 group are therefore taken into consideration, so there seems reason to hope that the 

 course of evolution here sketched out offers, at any rate, a basis for constructive 

 criticism . 



The Docoglossa exemplify the adaptation of the Prostreptoneure to a life more 

 especially in exposed parts of the tidal zone, coupled with the adoption of the method of 

 adhesion for protection. In the specialization to the adhesive habit is involved the loss 

 of the imperfectly developed method of protection by retraction, i. e. the reduction of the 

 operculum, and the cessation of evolution on the lines above sketched out for PleurO' 

 tomaria and the Trochidse. For efficient adhesion there should be an adhesive surface 

 broad in proportion to the height of the adherent body, a shell capable of being pulled 

 down and held down symmetrically, and a shape which would give the least possible 

 purchase to the waves. 



The Foot, already broad in front, was shortened by loss of the tail-process adapted for 

 a creeping habit, and was further broadened into an oval — a change which hastened the 

 disappearance of the habit of retraction. The animal probably possessed symmetrically 

 disposed shell-muscles, but if that symmetry had previously been disturbed, it was now 

 re-established. 



The Edge of the Shell now grew downwards and outwards, ever enlarging the rim so 

 as to form a complete cap for the animal. Stages of such a development have been 

 seen and figured by Boutan (5). It then became necessary to be able to pull down the 

 shell into contact with the rock or other surface by a force operating as symmetrically 

 as possible and as dii'ectly as possible at every point. Backward marginal extension of 

 the paired shell-muscles secured this to a large extent, and had the advantage that it 

 gave the greatest possible moment to the downward force. Ultimately, the extensions 

 met behind and gave the horseshoe form to the shell-muscle, forming at the same time 

 a wall around the soft viscera within (PI. 15. figs. 3-7). 



The fibres of the shell-muscle went down into the foot, the outer ones almost vertically, 

 so as to hold the shell-edge down, the inner ones obliquely, so tliat they exerted a 

 downward and inward pull on the shell-edge. The inner oblique fibres, which very 

 greatly strengthen adhesion, seem to characterize more especially the Cyclobranchs. 



The horseshoe (marginal) shell-muscle of the Docoglossa contrasts Avith the central 

 muscle of the Haliotidae, which pulls the roof-like shell down into close contact with 

 the contracted animal, without, however, bringing the shell-edge down against the rock. 

 A cap-shaped shell, which it was possible to draw down against the rock at every point, 

 must have imprisoned the epipodium within a narrow space and led to selection of 



