L70 C. D. WALCOTT — DISCOVERY OF SILURIAN VERTEBRATES. 



they show merely a stratification of lamella? lying one above the other, 

 while in vertebrates growth takes place from within by special cells. 

 odontoblasts or osteoblasts. The fossil proofs for the former are the den- 

 tine tubules : for the later, the outlines surrounding the osteoblasts. The 

 former we saw in the dentine tubercles, figures 1-3 : the latter are dis- 

 tinctly recognized in figure 4. which is enlarged to about 350 diameters. 

 It plainly shows small, irregularly bounded hollow spaces with ramify- 

 ing and anastomosing shoots. These I can only regard as true osteo- 

 blasts, peculiar to the hard dermal parts of the ganoids, inclusive of 

 placoderms. Their existence might at once be conjectured from the 

 outer appearance of the remains. Of course only detailed investigation 

 can show whether they exist in all the remains here described. In the 

 cross-section shown in figure 1 they appear to be preserved in the lower 

 parts, yet their state of preservation there is far less perfect, so that their 

 existence can merely be designated as probable. Briefly speaking, the 

 observations show the following facts: 



1. The existence of undoubted dentine tubules proves beyond doubt 

 that the remains, so far as they have been microscopically investigated. 

 belong to vertebrates. 



2. The occurrence of true osteoblasts distinguishes these hard parts 

 beyond doubt from those of the elasmobranchii and relegates them to 

 the division of the ganoids. Enamel could not be found in the speci- 

 mens studied. On account of this and by the strikingly distinct concen- 

 tric lamination in the dentine tubercles, the hard parts investigated indi- 

 cate a low stage of development. 



Professor James Hall:* In reference to the invertebrate fossils shown 

 me as coming in above the beds containing fish remains, I need only say 

 that they have a general Lower Silurian facies and represent in their 

 genera and species the fauna of the Trenton period, including Birdseye, 

 Black rivei-. and Trenton limestones. Some of them which were pointed 

 out as coming from the higher beds as exposed in the section seem tome 

 to be representatives of the Hudson River horizon ; for example, Orthls 

 (Plasseomys) subquadrata. The abundance and large size of the speci- 

 mens of Rhynchonella increbescens or R. capax seem scarcely compatible 

 with the strict limitation of the Trenton horizon. Comparing the lists of 

 the species which have been made, I can have no hesitation in coincid- 

 ing with the determinations, thus leaving no doubt whatever of the nature 

 and age of the deposits. 



* A note communicated to the author. 



