MUSEUM OF COMPARATIVE ZOOLOGY. 100 



Figure I. 



Deroceras plantcosta. Two septa showing the siphonal funnel, P; 

 the dorsal lobes, H ; and the ventral cell, J. 



Figure II. 



Deroceras planicosta. Shell of embryo, d', lining layer; X, external 

 layer ; d", median layers of the apex of first whorl, an internal transparent 

 and a thicker darker layer, both overlapped by X; d'", external layer of first 

 whorl next to dark layer of d" ; d, additional layers deposited by the dorsal 

 portion of the body of the animal in passing from neck of ovisac to seventh 

 septum ; le, first septum. 



Figure III. 



Nautilus Pompilius. An ideal outline of the ovisac and apex of first 

 whorl. A, ovisac; 1c, first whorl; W, shoulder formed by the external 

 ridge of the cicatrix ; X shows where the shell of first whorl connected with 

 the shell of the embryo, and also where it is always found broken. The two 

 dotted lines from X show the area of the scar itself as seen from the side ; T 

 is the line of involution showing the extent to which the apex is enveloped at 

 the first revolution of the shell. 



