176 J. E. S. MOORE. 



the co-existence in tliis species of a Naticoid organisation 

 with a Cerithio-planaxoid tonj^ue is exactly what, judging 

 from the foregoing observations, we should naturally expect; 

 for the complete retention of a Cerithio-planaxoid radula in 

 zonatus indicates, like the rest of this animal's anatomy, 

 that we have in this form what is probably the most primitive 

 Naticoid at present known. 



Concerning the Lamellarian nervous system of S. zonata 

 it has already been rendered evident, by the work of Haller,^ 

 how such a modification has been derived from the more 

 normal Gastropod types. To illustrate this I have diagram- 

 matically represented the nervous system of Sigaretus 

 neritoides, in which it will be seen that the shortening of 

 the subintestinal cord is very pronounced (PI. 17, fig. I). In 

 Natica lineata (PI. 17, fig. II), on the contrary, the nervous 

 system remains simply zygoneurous and bilaterally symmetri- 

 cal, with nothing remarkable about it except the simultaneous 

 shortening of the supra- and sub-intestinal cords. In fig. 4 is 

 represented the nervous system of S. zonata, and it will be seen 

 that the peculiarities which this form presents are primarily 

 due simply to a more complete shortening up of the supra- and 

 sub-intestinal cords. In fig. 5, which represents the nervous 

 system of Crucibulum, another line of modification is intro- 

 duced, for in this form it is the sub- and not the supra-intestinal 

 cord which is becoming short. A still further progressive 

 shortening of the subintestinal cord is witnessed in Trochita 

 radians (fig. 6), and the most extreme case of this progressive 

 development is represented in Crepidula. These modifications, 

 it will be observed, are the exact inverse of those obtaining in 

 Sigaretus (fig. 1). It is thus apparent that Sigaretus and 

 Crepidula represent the extreme terms of modification in 

 opposite directions of a mean type of nervous system, which is 

 exemplified by that of Natica lineata. 



We witness thus the very instructive fact that both these lines 

 of modification have been carried to opposite extremes in genera 

 which are unquestionably close allies. S. zonata (fig. 4) 



' Ijoc. eit. 



