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VII. On the Evolution of Topographical Relations among the Bocoglossa. By H. J. 

 Fleure, D.Sc, Fellow of the JJnicersitt/ of Wales. {Communicated by Professor 

 W. A. Herdman, F.B.S., F.L.S.) 



(Plates 15-17.) 



Read 17th December, 1903. 



JL HE researches, the results of which are here described, were pursued in the labora- 

 tories of the University College of "Wales, Aberystwyth, and owe a great deal to the 

 kind encouragement and stimulating advice of my friend, Professor Ainsworth Davis. 

 Prof. Yves Delage was so good as to allow me to work in the Lacaze-Duthiers Laboratory 

 at Roscoff during the summer of 1902, and I have to thank him for affording me 

 the possibility of collecting material and observing the habits of the animals studied. 

 I must also thank my friend Mr. H. N. Adair for very valuable help in improving and 

 finishing the sketches which accompany this paper. 



Part I. — The common Ancestor of the Prosobranch Gastropods. 



The Docoglossa form a well-marked group, having many characteristic features 

 possessed by all tlie members. Such features include the oval foot, the horseshoe- 

 shaped shell-muscle, the general characters of the visceral hump, the position and form 

 of pericardium and heart, the characters of the kidney, and the disposition of the gonad. 

 These features, therefore, seem to have been acquired before the members of the group 

 diverged amongst themselves, and it is thus of special interest to trace their history 

 from an origin somewhat farther back than their latest common ancestor. The 

 group retains several very primitive Gastropod characters, such as the close approach to 

 external symmetry, the symmetry of the shell-muscle, the strong labial commissure, and 

 the two kidneys each possessing excretory tissue and each communicating with the 

 pericardium. This allows the conclusion that Docoglossa branched off from near the 

 base of the Gastropod stem, and it is therefore best to take as starting-point the latest 

 common ancestor of tlie Prosobranch Gastropods. 



The present account of tliis hypothetical form is based upon inferences drawn from 

 a direct study of the detailed anatomy of various Docoglossa — Emarginula, Fissurella, 

 Haliotis, Scisstirella, and Trochns. I have also utilized, as far as possible, the results 

 of the work of Pelsenter, llaller, Boutan, Thielc, and Woodward on various archaic' 

 Gastropods. The form described is for convenience referred to as the Prostreptoneure. 

 This name is selected because the form is supposed to have already undergone that torsion 

 of the branchial region and visceral hump which characterizes the Prosobranclis, and 

 therefore to have possessed the twisted visceral loop of the nervous system. 



It is, however, possible that the Gastropods had begun to diverge among themselves 



SECOND SERIES. — ZOOLOGY, VOti. IX. 38 



