president's address. 71 



cerebration, for, wishful to enquire further into the evolution of the 

 Docogiossa, I re-consulted the interesting paper by Dr. Fleure (16),^ 

 and found, what had escaped my memory, the germ of the idea there. 



Dr. Pleure hypothecates a prostreptoneure ancestor for the 

 Prosobranchia, and reconstructs and figures such an animal. This 

 prostreptoneure was in his opinion probably far more symmetrical 

 externally than many of its descendants, with a symmetrical pair of 

 shell-muscles, and nearly, but not quite, symmetrical shell possessing 

 a moderately developed spiral, coiling in or near the sagittal plane, 

 while in its anterior edge there was a sinns or slit. It had also a 

 moderately developed operculum. Among other points in support of 

 his contention. Dr. Fleure directs attention to the fact that " among 

 the earliest Gastropod fossils we find many feebly spiral shells which 

 are almost or quite symmetrical" {16, p. 270). 



Personally I would accept Dr. Fleure's Prostreptoneura, and have, 

 therefore, included the name in the tables, but I would define the 

 animal and shell as perfectly symmetrical, coiling in the sagittal plane, 

 with a complete operculum, and regard the loosely-coiled, capuloid 

 shells found in early strata and usually referred to Platyceras as their 

 modified descendants. Nor do I think the presence of the slit 

 necessary, for, as XJlrich points out {52, p. 948), there is an almost 

 total absence of a long, parallel-edged slit in the lower Silurian 

 Pleurotomariidae, while according to Hall, who also is not speaking of 

 the oldest forms {20, p. 16), there is in many species of Platyceras 

 a sinuosity of the striae indicating a notch in the margin of the 

 aperture during the first stages of growth, which does not always 

 persist in the adult stage. In those in which this notch becomes 

 closed another begins at some other point, while in others the 

 peristome becomes plicated with several sinuosities in the mature 

 condition. Seeing that in life most of the genus attached themselves 

 to foreign bodies, these various sinuosities were probably due to 

 irregularities on the surface of the object of attachment, and do not 

 reflect any important anatomical structure in the animal. 



Unfortunately the casts of these fossils do not so far appear to have 

 yielded traces of the muscular attachment, and it is therefore not 

 possible to say whether two distinct scars exist, or the single horseshoe- 

 shaped scar of the Capulidoe. Ivoken describes and figures {2Ij., p. 464, 

 pi. xi, fig. 9) a cast under the name of Platyceras Protei, CEhl., from 

 the Lower Devonian, showing the capuloid muscle-mark, but this 

 cast obviously appertained to a shell without any spire and came from 

 a far higher horizon than those of which we are speaking. Moreover, 

 once the spire disappears, the strengthening of the muscle follows 

 as a matter of course, just as in the Limpets, which equally have 

 a horseshoe-shaped muscle attachment, but are not on that account 



1 The author desires me to mention in citing this paper that the impression 

 accidentally conveyed here and there in it that he included certain of the Rhipidoglossa 

 among the Docogiossa is due to an unfortunate oversight when passing the proofs, as 

 the conteit of the whole paper shows. 



