7 



broad waves or corrugations. The lines of growth are 

 rather indefinite striae, not nearly so distinct as the ridges 

 of the first system, except near the mouth of the shell 

 and particularly on the shell siphon. The corrugations 

 on the other hand are much more definite on the earlier 

 whorls, and on the last or mouth whorl of an adult shell 

 they are only well marked near the suture line. 



These transverse corrugations relieve the monotony 

 of the plane surface and are one of the first characters 

 which strike the observer when comparing the shell with 

 that of Fusus. 



A longitudinal section taken through the body whorl 

 of the shell shows the following structure : — (a) an outer 

 wide layer of irregular columns; (b) a middle and 

 narrower layer, also composed of columns, which are, 

 however, regular in shape and arranged at almost a right 

 angle to the surface of the shell; (c) an inner layer 

 characterised by delicate oblique cross lines. 



The outermost layer begins at the outer lips of the 

 shell mouth, the middle layer commences a little further 

 inside, and it is soon followed by the inner layer. 

 According to Tullberg, who seems to have made a careful 

 study of the structure, a fourth and more internal layer 

 still, occurs in the older whorls and increases in thickness 

 as one approaches the apex of the shell. This is not seen 

 in the micro-photograph, which is from a section through 

 the wall oi the body whorl. Text-fig. 2, after Tullberg, 

 indicates the position of origin of the layers. 



In addition to these layers of shell substance, there 

 is a very well developed periostracum which can be quite 

 easily peeled off from the shell and examined without 

 sectioning. 



The Periostracum is a chitinous layer, yellow in 

 colour, and raised on the external surface into a number 



