DIFFERENCES IN HISTOLOGICAL STRUCTURE OF TEETH. 463 



network characteristic of the ling and hake, and that within 

 the limits of this single genus many stages in the reduction of 

 the vascularity are exemplified, although in none has it wholly 

 disappeared. 



In the burbot (Lota vulgaris), the only fresh-water 

 representative of the family, there is an interesting arrange- 

 ment. This fish is generally regarded as closely allied to 

 the ling, of which it is sometimes spoken of as the fresh- 

 water representative. In it the vascular channels are few, and 

 the upper part of the tooth is generally free from them, but 

 where they do exist they present the flattened exteriors of the 

 loops. They thus recall the circumferential canals of the ling, 

 and differ from those teeth found in the genus Gadus, in which 

 the canals are reduced to a similar extent. So far, then, 

 as dentine structure goes, the relationship of Lota to the 

 ling appears to be somewhat confirmed (fig. 11). A similar 

 but richer disposition of vascular canals occurs in Brosmius 

 (fig. 3). 



In Lotella, usually regarded as allied to the hakes, the vas- 

 cular canals have utterly disappeared, and the dentine presents 

 no structure save a well-marked lamination (fig. 12). 



To sum up the differences which have been so far described, 

 they are of two kinds : the one the diminution in abundance 

 of the vascular canals ; the other a slight difference in their 

 arrangement, consisting in the absence of the distinctly 

 bounding canal which is formed in the cases of the hake and 

 ling by great flattening of the exterior loops and their anasto- 

 moses with their neighbours. In each case, therefore, it is a 

 difference of degree rather than of kind ; nevertheless no one 

 could possibly mistake the teeth of the hake or of the ling for 

 those of the genus Gadus ; and, as has been already pointed out, 

 the difference in the form of the loops is still perceptible, 

 even when they have become greatly reduced in number, as in 

 the burbot (Lota). 



The teeth which I have examined are those of the following 

 genera :— Gadus, Merlucius, Molva, Lota, Lotella, Uraleptus, 

 Phycis, Motella, Raniceps, and Brosmius ; whilst of the genus 



