412 Miscellaneous. 



The Phylogeny of the Docoglossa. 

 By W. H. Ball. 



In his concluding- fasciculi, contributed to complete Troschel's 

 classical ' Gebiss der Schnecken,' Dr. Johannes Thiele dissents very 

 emphatically from some suggestions of mine in regard to the deriva- 

 tion of the true limpets, made many years ago. At that time it 

 appeared to me that the Lepetidse might represent the stem, some- 

 what degenerated, from which the Docoglossa were derived. While 

 I attach, even in the present state of our knowledge, comparatively 

 little importance to speculations of this kind, which can only be 

 placed on a firm footing by extended embryological researches, it 

 still seems to me that there is a solid basis for the hypothesis which 

 I then suggested. 



There can be little doubt that the early type of Gastropod gill was 

 situated much as in Fissurella, on the " back of the neck " behind 

 the head, and that it was constituted of a stem with lateral lamellae. 

 Originally paired and symmetrical, by circumstances incident to 

 growth and torsion one gill of the pair has in most cases become 

 aborted, though its " smelling organ " frequently remains, as in the 

 limpets. There is also no doubt whatever that the protolimpet was 

 derived from a form having a spiral shell. I have shown that Pro- 

 2)iUdium by its dentition is closely allied to Lepeta. Now Propi- 

 lidium is said to have two gills, but certainly has at least one, of the 

 type of Acmcea. It retains a spiral nucleus through life, though it 

 is partly cut off by a small septum, which is never completed. Other 

 Lepetidae also show a spiral nucleus when very young, but it is cut 

 off completely and lost later. These other, mostly deep- or cold-water 

 forms, have lost their gills and eyes by degeneracy, and the principal 

 teeth of the radula show a tendency to become cemented together, 

 while in Propilidium they are more or less isolated. Now in the 

 Acma^idae and Patellidoe the nucleus is limpet-shaped from the 

 beginning ; the uncinal teeth (well developed in Lepeta) are 

 degenerate and often lost in the Acmaeas, but appear again in the 

 Patellas, not, however, with the individuality and completely chitin- 

 ous nature which is found in the corresponding teeth of Lepetidae. 

 We find therefore in Lepetidae the greatest number of archaic cha- 

 racters (somewhat masked by degeneration of other organs) which 

 remain in any of the three groups, and, whether most ancient or not, 

 so far as these characters go the Lepetidae are nearest to the proto- 

 limpet. 



In my work on the ' Blake ' mollusks (i. p. 436) I said that Acmaeidse 

 of all the groups of Docoglossa is the most typical ; that is, within 

 the limits of that family are found assembled, sometimes in one and 

 the same animal, the greatest number of organs which, taken singly, 

 are characteristic of Docoglossa. This is strictly true ; but Dr. Thiele 



