MORPHOLOGY OF THE HELICINID^E. 803 



wrong, I fail to see that the conclusions deduced from it by Dr. 

 Simroth are necessary. I will explain as briefly as possible why. 

 Unless I misunderstand him grievously, and if I do I beg his 

 pardon, one of Dr. Simroth's chief conclusions is that, contrary to 

 the generally accepted doctrine, marine and freshwater animals in 

 general, the marine prosobi-anch Gastropods in particular, are 

 evolved from terrestiial forms which have been forced by the 

 above-mentioned secular inundations to adapt themselves to new 

 conditions of life and make their habitat in another medium. As 

 the pendulation theory applies to all geological time, if the pre- 

 cursors of marine Gastropods were terresti"ial in habit, we should 

 find evidence of this in geological deposits. The earliest-known 

 Gastropods, from the Cambrian to the Devonian, would bear 

 evidence of their terrestrial life, those found in later deposits 

 would indicate, in some periods at least, the change from a ter- 

 restrial to a marine existence. But, in point of fact, the geological 

 evidence points decisively the other way. In Cambrian, Ordoviciaii, 

 Silurian, and Devonian deposits we get Gastropods belonging 

 almost exclusively to the Streptoneurous Aspiclobranchia and 

 Pectinibranchia. There are, it is true, the pteropod-like shells of 

 the Conularida which, if they are really remains of Pteropods, 

 would demonstrate the great antiquity of highly specialized forms 

 of Euthyneura. But the true systematic position of the Cornu- 

 larida is at the best doubtful, and it has been urged with much 

 reason and on high authority that the resemblance between the 

 shells of these archaic forms and the more modern Pteropoda is 

 due to parallelism. As so much doubt prevails as to their atSnities, 

 the Cornularida cannot be bi-ought into the argument. The 

 Aspidobranchiate and Pectinibranchia te Gastropods from these 

 earlier Palaeozoic deposits are without doubt marine forms. They 

 subsisted, without a,ny important changes, through the four above- 

 mentioned geological epochs, and one genus, Pleibrotomaria, has 

 survived to the present day. We know the habits and the 

 anatomy of Pleurotomarla, and they support in a most remarkable 

 manner the conclusions derived from an extensive knowledge of 

 gastropod morphology. On the other hand, with the exception of 

 Hercynella from the Devonian, undoubted Euthyneura first make 

 their appearance in the Carboniferous, They belong to the 

 Actfeonidae and Pnlmonata Stylommatophora. The first-named 

 family is marine, and anatomically displays so many strepto- 

 neurous characters that it might almost be included in the 

 Aspidobranchia. Of the Stjdommatophora we get forms like 

 Dendropu'pa and Pyramidula, unquestionably terrestrial species, 

 and, according to views generally accepted, highly modified and 

 therefore indicative of a line of lost ancestry probably allied to the 

 contemporary Actteonidse. But these pulmonate forms are few 

 and of rare occurrence in the Cai-boniferous, a period in which 

 the conditions for the preservation of terrestrial and freshwater 

 forms were pai'ticularly favourable. Had numerous Pulmonates 

 existed at that time their remains must have been mure abundantly 



