THE UPPEK SILURIAN OF NOETH AMERICA. OO 



and left of them, but all direct their course more or less obliquely to 

 the upper, outer surface, where their finest branches detail out and 

 end. They perhaps form a communication with the outside ; but I 

 have not been able to see their terminations. In some sections it 

 appears as though an extremely thin transparent layer existed, 

 covering and sheathing the whole, but of this I am not absolutely 

 certain. 



The agreement between the above description and Prof. Huxley's 

 account of Scaphasjns Lloydii is so close that no reasonable doubt 

 can be entertained regarding their close relationship. At the same 

 time certain distinctions exist which, considered in addition to those 

 of external form and markings, have prevented my placing it in the 

 same genus. 



Judging from Prof. Huxley's figures, and the examination 

 which I have been able to make of the structure of the English 

 Scaphaspis, the vessels in the reticular layer pursue a very irregular 

 course ; hence this name. This is not the case in the fossils under 

 consideration. Their vascular system (fig. 4) exhibits a regularity 

 and parallelism, of which the figures of Scapliasjns afford no trace. 

 This alone instantly differentiates the two. Moreover, the beautiful 

 forest-like appearance of the tubules radiating from the vessels is 

 much more marked in consequence of this regularity than in the 

 engraving of Scaphaspis. Lastly, I have seen no trace of the 

 crenulation on the edge of the superficial stride of which Professor 

 Lankester makes mention. 



These, with some minor considerations, induce me for the pre- 

 sent to refrain from placing these Pennsylvanian fossils in any of 

 Lankester's genera, and T propose therefore to refer them to a new 

 provisional genus to which I have given the name Pal^aspis*. 



Another point must not be omitted, because it forms to the anato- 

 mist one of the most remarkable features in the structure of these 

 shields. In none of them, either English or American, has any 

 trace been discovered of true osseous tissues. The bone-cells or 

 lacunce, and the tubules or canaliculi connecting them are totally 

 absent, unless the minute tubular system above described be con- 

 sidered representative of the latter. This remarkable fact was one 

 of the chief, perhaps the chief objection to admitting the claim of 

 these fossils to a place among the Yertebrata. But the objection has 

 been overruled, and consequently the line separating the Vertebrates 

 from the Invertebrates rendered to that extent less distinct. 



Professor Lankester has, in his monograph, established the sub- 

 family "Heterostraci" to receive those forms devoid of true bony tissue. 

 He has thus divided Huxley's family of Cephalaspids into two parts, 

 retaining in the higher of tli£se (the " Osteostraci ") all those forms 

 which, like CepJicdaspis, possess bone-cells and tubules (lacunce and 

 canaliculi). 



So profound an anatomical distinction merits, it appears to me, 



* The name Gli/ptaspishas appeared by oversight in one or two preliminary 

 notices in magazines in reference to these fossils. Its occurrence was accidental, 

 and it mast therefore be considered merely a synonym. 



