848 PROF. G. C. BOURNE ON THE [ISTov. 17>. 



lobe, but in conti'acted specimens this end always appears to be 

 posterior in position. The siibequal development of the right, 

 and left columellar muscle is without doubt related to the 

 peculiar development and functional importance of the semilunar- 

 operculum, the straight and moi-j)hologically anterior edge of which 

 works against the anterior edge of the so-called area in a hinge- 

 like manner. When the animal is fully extended the operculum 

 is turned back and pressed close to the flat surface of the area ; 

 and an examination of a relaxed specimen or of a living Neritina 

 ihiviatilis shows that it is a mechanical necessity that there 

 should be an equal pull at both ends of the semilunar operculum 

 in order to bring about an efiective closure of the aperture when 

 the animal retreats into its shell. There can be no doubt that 

 the operculum is a very important factor in the economy of the 

 Neritidse, and that, in the course of evolution, its growing 

 importance has had much to do with the tendency to reduction 

 of the visceral spire exhibited by members of this family. It 

 can hardly be doubted that the physiological effect of a well- 

 developed left shell-muscle, exerting a jjull upon the oi'gans of' 

 the left side every time that the animal withdraws itself into its. 

 shell, will be to draw those organs over from I'ight to left and 

 thus to uncoil the typical gastropod spiral in opposition to the 

 action of the right muscle. When both muscles are equally 

 developed and inserted symmetrically on the interior of the shell, 

 as is the case in existing Neritidfe, their joint action must tend to 

 the establishment and maintenance of a symmetrical arrangement 

 of such organs as are affected by their action, and we find, in 

 fact, that the disposition of the two muscles produces an 

 appearance of symmetry in the region of the head and foot,, 

 which to a certain extent affects the organs of the pallial complex 

 and of the proximal part of the visceral mass. It is known that- 

 in Fissurella the secondary symmetry of the adult is acquired in 

 the course of development by the relatively great growth of the- 

 foot, the formation of a shell-muscle on the left as well as on 

 the right side of the body, and the subsequent rapid gi'owth 

 of the right side, causing a shifting back of organs from right 

 to left. But in this case the increased functional importance of 

 the sole of the foot and the loss of the ojDerculum and opercular 

 lobe have been the main factors in determining the final relations 

 of foot, shell, and visceral mass. In the Neritidte different 

 conditions have pi-evailed. The sole of the foot I'emains relatively 

 small ; the operciilum and opercular lobe become large and 

 important, and by their presence prevent any posterior union of 

 the right and left shell-muscles ; the visceral spire remains, 

 connected with the cephalo-podial regions by a relatively nairow 

 pedicle. The different lines of evolution are well illustrated by 

 Septaria, in which the sole of the foot has enlarged so much that 

 it rivals that of a Fissurella, a Patella, or a Gajndus in size and 

 importance, and the shell has become almost patelliform, but the 

 operculum, though functionless, retains its place between the 



