270 DE. H. J. PLEUEE ON THE EVOLUTION OF 



before that process was complete, and the Prostreptoneure is to be regarded as a 

 general type of what were probably a few closely related forms. 



The Prostreptoneure was probably far more symmetrical externally than many of its 

 descendants. This view is supported by the following considerations : — 



1. The Docoglossa remain symmetrical throughout development, and at an early 



stage Haliotis has a symmetrical pair of shell-muscles [Botitan). The earliest 

 j)ost-torsional condition of the shell-muscle is almost certainly a paii'ed one, and 

 that of Scissurella, which shows this condition, may be primitive. 



2. Among the earliest Gastropod fossils we find many feebly spiral shells which 



are almost or quite symmetrical. Among the oldest Gastropod fossils also is the 

 large order of the Bellerophontidse, which usually possess symmetrical shells. 



3. The remains of symmetry, both external and internal, are far more marked among 



tlie more primitive than among the more specialized Prosobranchs. 



The torsion, however, had so profoundly disturbed internal symmetry that the retention 

 of complete external symmetry is improbable. 



It is therefore supposed that the shell of the Prostreptoneure was nearly, but not 

 quite, symmetrical and possessed a moderately-developed spiral, coiling in or near the 

 sagittal plane. In the anterior edge of the shell there was a sinus or slit, which was 

 situated in the median plane or somewhat to the right. 



The Poot was fairly long and primarily useful for creeping over rocks and on seaweeds. 

 It probably possessed a broad, and possibly bUobed, front edge, an adaptation to the 

 habit of creeping upon the surface of the water. This breadth may have also 

 permitted swimming-movements to a moderate extent. There was a moderately 

 developed operculum on the postero-dorsal surface of the foot. 



The approach to external symmetry indicates, perhaps, a certain ability to move 

 through water, probably by swimming and surface-creeping. It is also adapted to the 

 circumstances of a life on floating seaweeds, but the Prostreptoneure was more probably 

 a shore-living form, perhaps favouring among other places the rock-pools. The marked 

 asymmetry of Pleurotomaria, the Trochidse, and typical Monotocards seems to have been 

 developed as a further specialization to a creeping habit. 



The Alimentary Canal was in the form of a u with some amount of extra coiling on the 

 distal limb of the U- This can be inferred with some certainty, as a gut of that type 

 occurs in Tleurotoynaria, Haliotis, many species of Trochus, and various Tsenioglossa, 

 and it also characterizes the Cephalopoda. The general disposition of the gut will 

 be further discussed in dealing with the consolidation of the Docoglossan visceral hump 

 (see PI. 15. fig. 6). 



Within the lips the lining epithelium of the mouth was cuticularized, and there must 

 have been a pair of lateral projections bearing extra strong cuticle which formed the jaw- 

 plates. These jaw-plates were probably united by a median dorsal piece, but the arch 

 with the strong front edge seen, for example, in the Patellidse is perhaps a specialization, 

 though it certainly resembles the upper jaw of Cophalopods, as Thiele has remarked. 



It is certain that the Docoglossan odontophore has been greatly specialized during 

 the evolution of the group, and one can consider the relations in Pleurotomaria as 



