REPORT ON THE PTEROPODA. 
91 
ciniclse. We know that in these latter also the sinistrorsity has only affected the coiling 
of the spire. 
It is easy to explain the transition from a creeping Bulloid to a swimming Limacinid. 
Even among the Bulloidea we observe a great tendency to natatory habits ; the margins of 
the foot (parapodia) extend laterally so far that they can be reflected over the shell, and 
assist by their movements, in a natation at first imperfect then gradually more complete, 
in the forms which have become more specialised ( Acera , &c.), and even carried out to a 
very high degree in Gastropteron. 
It is quite comprehensible how, among animals having such tendencies, forms should 
have arisen having the mantle and shell well developed and with sinistra! coiling, which 
by gradual specialisation have become exclusively pelagic animals, the first rough sketch, 
as it were, of the Limacinidae. 
If we examine the whole series of the Bulloidea (or Cephalaspidea), living and fossil, 
we shall find that the most ancient are forms resembling Actseon (these are probably the 
most ancient of the Opisthobranchia, and their importance with respect to the phylogeny 
of the Gastropoda cannot be overrated) ; the organisation of the recent Actseon (especially 
its nervous system, generative organs, and operculum), and its possible relations with the 
Pyramidellidae, show that it may be not very far removed from the common stock of the 
Streptoneura (Prosobrancbs and Heteropods) and the Euthyneura. 
The genus Bulla, however, properly so called, scarcely appears before the Cretaceous 
period. 
The presence of an operculum in the most primitive Thecosomata (Limacinidae and 
the larvae of the Cymbuliidae) shows that they are descended from operculate ancestors. 
Actseon still retains this operculum (it is the only Opisthobranch which does not lose it in 
the adult state), and all the fossil Actaeonidae certainly possessed it. The earliest Bullidae 
— sens, lat., i.e., comprising the Scaphandridae and the Tornatinidae — (derived from the 
Actaeonidae) must have possessed it also, and the animals of this family will only have lost 
it subsequently in the adult condition. It is from some of these operculate forms, inter- 
mediate between Actseon and Bulla, that the first Thecosomatous Pteropods have arisen. 
If for example we consider such forms as Globiconclui or Hydatina ; if we allow that 
some of them have become coiled sinistrally whilst retaining the dextrorsal asymmetry 
in their organisation (as happens in some Gastropods, e.g., Lanistes)-, lastly, if in these 
animals the lateral margins of the foot, already strongly developed, become still more 
specialised, we shall have the first Limacinidae. 
A sinistral shell from one of the forms above quoted would, in fact, closely resemble 
a short-spired shell of one of the Limacinidae, such as the earliest Eocene Limacinse. On 
the other hand, owing to more and more exclusive adaptation to pelagic life, the shell 
of the Bulloidea must have become more delicate, and have acquired a structure very 
similar to that of Limacina, as in the case of the shells of the living Haminea and 
