206 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



It should be noted that the unci do not attain their full development 

 till they reach the middle of the radula ; up to that point thej' 

 continue to be packed close to a layer of cells which is continuous with 

 the upper cells of the radular sac, and which continually adds to their 

 thickness by contributing more chitin. Here I ought to confess that 

 my information on the subject of the chemistry of chitin is still 

 rudimentary : to some extent I have been able to verify Zander's 

 experiments,^ which make it probable that chitin is a carbohydrate 

 closely related to glycogen. If this were confirmed by chemical tests 

 of some other kind, the result would be of great value to the student 

 of chemical physiology in the moUusca. The chemistry of glycogen 

 and its derivatives probably contains the secret of the snail's power of 

 hibernation, and upon this, I think, depend the extraordinary modi- 

 fications of the reproductive organs and processes with which we meet 

 in this group. 



Each uncus, as it leaves the matricial region, has a well-defined 

 apex. This is that part which I have hitherto shown as the lower 

 extremity of each ' tooth ' in my drawings. For the reasons now 

 apparent I shall treat it as the upper extremity. In many figures of 

 radulse it is omitted altogether. 



The apex is usually provided with a furrow [fossa apicalis) into 

 which the hinder part of the preceding uncus fits. It is further 

 furnished in most cases with a vertical notch, a continuation upwards 

 of the fossa. 



The underside of the uncus is flattened and forms the basal plate ; 

 it becomes afiixed to the hasal inembrane, or else is in some way, not 

 yet well understood, imbedded in it. But the basal membrane shows 

 no sign of being converted into chitin. I have applied Zander's tests 

 for chitin to denuded pieces of basal membrane without result. 

 Staining processes especially designed to act upon chitinous materials 

 do not colour the basement membrane. 



The hinderpart of the basal plate is generally somewhat wing- 

 shaped, and it exhibits a line, more or less parallel to the transverse 

 axis of the radula, approximately in the position which in a 

 bird's wing is occupied by the outer edge of the primary coverts. 

 Examination of isolated unci shows that this line is the edge, often 

 thickened and adhering secondarily, of an actual fold : a relic of the 

 original subconic configuration of the uncus. If one imagines a paper 

 cone so folded that its margin is pointed in front and more or less 

 squared at the back ; two slits made in the sides of the cone, between 

 front and back ; the pointedness of the front margin increased by 

 infolding along the sides of the slit, and a slight fold made on one or 

 both sides of the squared part of the margin ; the thickness of the 

 point and other folded parts increased by dipping in melted paraffin, — 

 a fair idea may be obtained thus of the building up of the uncus. To 

 this fold on the basal plate I wish to give the descriptive term lacinia. 

 It appears that in most cases the lacinia forms a kind of articulation 

 with the fossa apicalis of the next uncus, so that the bases of the unci 



1 Pfliiger's Arch. 66, 545 (1897). 



