384 Mi- Tims, A suggestion as to the nature of the 



is to say, that the formation of additional calcified structures 

 takes place in the groove where the scale-pocket and the fibrous 

 basis become continuous. Comparison of the scales of different 

 genera and species and with those of the Clupeoids show 

 that there is a tendency to a fusion of the scalelets together with 

 a disappearance of their projecting points, tending to form a 

 continuous and smooth-surfaced calcified plate. Moreover, the 

 development of the scale is seen to take place in the more super- 

 ficial layer of the dermis, which is composed of elongated nucleated 

 cells with ill-defined boundaries, and so closely set that I have 

 spoken of it [4] as the " nuclear layer." Beneath this is a 

 more fibrous-looking stratum with but few elongated nuclei. 

 Lying superficially to these two layers is the epidermis with its 

 mucin-secreting glands. The epidermis is readily detached and 

 usually absent in preserved specimens. 



If this account of the structure and development of the tele- 

 ostean scale be compared with that given by Beard of the genesis 

 of the horny teeth of Petromyzon, I think it will be seen that 

 there is more than a superficial resemblance. Beard says that 

 " the mouth is lined by a thick stratified epithelium. Lying in 

 the deeper layers of this epithelium, that is forming its basis, one 

 notices a somewhat symmetrical row of what, at first sight, looks 

 like true tooth-sacs." " Though they possess a dental papilla they 

 have no odontoblasts." And again, " The papilla is mesoblastic 

 [of reticulate connective tissue], and upon it there rests an 

 ' enamel epithelium ' which passes over as in other cases, into the 

 outer epithelium. The curious circumstance is that from the start 

 the enamel epithelium and the circular horn groove into which it 

 passes externally produce horn, and beyond the imitation of a true 

 tooth-sac in which, if ever present, the odontoblasts never become 

 functionally active, there is no trace of true tooth structure ever 

 produced in Petromyzon planeri." 



This description applies only to the teeth near the anterior 

 margin of the mouth, those situated more inwardly " are formed 

 simply in the basal layers of the stratified epithelium, without the 

 intervention of a tooth-sac." Now if we substitute for " stratified 

 epithelium " what I have termed the " nuclear layer," the con- 

 ditions appear to me to be almost identical. The true epidermal 

 covering is extremely liable to be removed, indeed by merely 

 placing the freshly killed animal fish into a preservative fluid, this 

 layer becomes detached, and I have not yet seen it present in any 

 preserved stock material. It seems therefore not unlikely that 

 this layer, even if present in the mouth, when the animal is alive, 

 may be similarly liable to become detached, and therefore that the 

 true epidermal layer is absent in the specimens examined by both 

 Beard and Warren. 



