492 Dr. J. 8. Hyland—Zonal Structure in Olivine. 
Dipnoan, it is certainly strange that he should represent it with the 
obtusely lobate pectorals of the Rhizodonts and Osteolepids instead 
of with the acutely lobate or unabbreviate archipterygeal members 
of Ceratodus or Dipterus. Except the mandible, no bones are marked 
off on the head, in accordance with the idea that the cranial roof is 
represented by a single dermal bone, while near the end of the snout 
are placed two minute orbits. It is to be hoped that no future 
compilers of geological or paleeontological text-books will copy this 
extraordinary figure and insert it as a ‘restoration of Dendrodus.” 
As the result of his researches Dr. Rohon announces that the 
genera Dendrodus, Owen, and Cricodus, Agassiz, are not Ganoids, 
and considers that they should be reckoned as forming a peculiar 
order of Dipnoi. Of this supposed “order” he gives the following 
definition :—‘‘ Fishes with depressed head, whose surface-ornament 
consists of ridges and tubercles. The free margin of the snout is 
set with numerous small teeth, whose folded dentine encloses a 
Spacious pulp-cavity. Two powerfully developed palatal teeth, 
which are intimately united with the pterygo-palatine, and within 
the folded dentine contain a spacious pulp-cavity; they are placed 
asymmetrically. The bony quadrate is fused with the skull. A 
styliform parasphenoid displays on its surface a tract of little tubercle-— 
like teeth, and is the bearer of several fused vertebrze, drawn 
into the skull. Very small orbits (?) and two internal nasal 
openings are present. The skull-roof represents a simple dermal 
bone, whose histological structure corresponds to that of true 
osseous tissue.” 
Now as this definition is founded upon an entire misinterpretation 
of the specimens figured in the paper, I fear it can only be looked 
upon as a curiosity of scientific literature. Whatever relationship 
“ Dendrodus” may have with the Dipnoi, it is certainly not in the 
manner indicated by Dr. Rohon, and I for one do not as yet see 
any reason for separating the Holoptychiide, of which I consider 
“‘ Dendrodontide ” to be a simple synonym, from the Crossoptery- 
gian Ganoids. 
Again, I must refer to my paper on the nomenclature of the Old 
Red Sandstone fishes with regard to Cricodus, which Dr. Rohon also 
includes in his Dendrodont Dipnoi. If Pander’s Polyplocodus be 
Synonymous with Agassiz’s Cricodus, then the latter is certainly 
not a Dendrodont or Holyptychian, but a Rhizodont. 
V.—On Zonat Srructure in OLIVINE. 
By J. SHearson Hytanp, Ph.D., M.A., 
Of Her Majesty’s Geological Survey. 
N a recent paper on the volcanic rocks of Kilimandjaro I referred 
to the presence in the basaltic lavas of olivine crystals, which 
appeared to possess zonal structure. Although actual proof was 
wanting, still the symmetrical arrangement of the inclusions, as also 
the mode of alteration of the crystals, rendered the occurrence of 
such a structure highly probable. The olivines, being rich in iron, 
were altered into ferric oxide or ferric hydrate; and the line of 
