180 rUOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



while the huihl distinctly reminds oneoi Pleurocera, s.s. Conchologically 

 the adults of these three form an almost complete transition from 

 Fleurocera on the one hand to Amhloxus on the other, yet there is 

 no reason to regard them as anything but generically distinct. To 

 the systematists, following out the idea embodied in Pelseneer's 

 classification of the Pelecypoda, this may doubtless seem irrational ; 

 a simpler arrangement would be to group all the species in Fleurocera 

 as the oldest generic name. Well and good, but Fleurocera represenls 

 a stock which has in it the latent possibilities of developing a fusiform 

 canal, Go7iiobasis a pleurotomariform cleft, while the line of modi- 

 fication of Ambloxiis is not known, but would doubtless involve some 

 similar modification of the aperture. In other words, these represent 

 three closely allied stocks modifying in an analogous but not a homo- 

 logous manner. The similarities are due to the fact that they have 

 reached the same stage of specialization. It has been noted in 

 ITelisoma that a round-whorled stage is succeeded by a sculptured stage 

 (carinate in that particular instance), and that in turn by a second 

 round-whorled stage, while in the allied Flanorhis a sculptured stage 

 (carinate) is succeeded by a round-whorled, that in turn by a second 

 sculptured stage (dentate), and that by another round-whorled stage. 

 This alternation of sculptured and round-whorled stages is as 

 characteristic of Gastropod evolution as the ever-increasing complexity 

 of the lobing of the Ammonoids or the migration of the umbones to 

 a terminal position in the Pelecypoda. These sculptured Gastropod 

 stages invariably form a key to the relationships of species, but the 

 round-whorled stages may not. In the Helicoid land shells, for 

 instance, several families so closely resemble one another that it has 

 only been with the study of the anatomy and embryonic whorls in 

 recent years that even an approximation of relationships has been 

 established. Wheii the development of each species is studied 

 carefully, numerous additional changes must be expected. 



Of course, the anatomy should be examined to confirm the separations 

 into Fleurocera, Ambloxus, and Gotiiobasis. But the fact that the 

 anatomy is unknown is no excuse for the sort of guesswork that has 

 pervaded this group. 



The genus is not certainly known in the fossil state. 



Gyrotoma (GoNioBAsis) oLivuLA (Courad). 

 Melam'a oliiuht, Conrad, 1834; Jl. cyllndracea, Conrad, 1834; 

 M. (squalis, Haldeman, 1841 ; 31. impressa, Lea, 1841 ; M. fusi- 

 formis, Lea, 1841 ; M. crebristriata, Lea, 1841 ; 31. Hayes^iana, 

 Lea, 1842; 31. Vanuxemiana, Lea, 1842; 31. protea., Lea, 1845; 

 31. auriculcejormis, Lea, 1845; 31. harpa, Lea, 1845; 31. basalts. 

 Lea, 1845; 31. arciata, Lea, 1845 ; 31. ccElatura, Conrad, 1849 ; 

 31. oppii(/nata. Lea, 1852 ; 3L clara, Anthony, 1854 ; J/, textilosa, 

 Anthony, 1854; 31. pupoidea., Anthony, 1854; 3[. crislata, 

 Anthony, 1854; 31. arnpla, Anthony, 1854; 31. ambuda, 

 Anthony, 1854; 31. abucida, Anthony, 1860; 31. decorata, 

 Anthony, 1860 ; 31. grata, Anthony, 1860-^ 3T. lachri/ma, ' Anthony 

 MS.,' Kceve, 1861; 3[. varians", Lea, 1861; 31. blanda, Lea, 



