246 6 © B.’"NATURAL HISTORY. 
they originated in still more generalized types. This accords with 
Huxley’s view of the period of origin of the Mammalian orders. 
It must also be remembered that the above deduction as to 
geological distribution is precisely that of geographical distribu- 
tion; 7. ¢, that the homologous groups of different continents are 
not ovale, but subordinate divisions of orders, the orders being 
universally distributed. This coincidence is remarkable, and justi-. 
fies the view I have taken of the origin of higher types on the basis 
of retardation and acceleration, and of the nature of synchronism.* 
Note in reply to Dr. Seeley’s remarks on my interpretation of the structure 
of the cranium of Ichthyosaurus. . 
A brief abstract of the portions of the preceding paper, which relate to 
Ichthyosaurus and Lystrosaurus, having been published in the ‘‘ American 
Naturalist,” for 1870, Dr. Secley publishes a criticism of the statements and 
conclusions therein contained, in the ‘‘ Annals and Magazine of Natural 
History ” for April, 1871. I will briefly reply to these remarks; and com- 
mence by saying that he discovers some errors in determinations of bones of 
the cranium of Ichthyosaurus, which are due to errors of the artist and proof 
reader; such are more likely to occur in an abstract issued early in a 
periodical, than in the essay itself. Thus he finds the lettering of the maxil- 
lary and lachrymal bones to be exchanged. This, as he supposes, is the 
artist’s error, and one which was corrected on the proof which was not 
received in time. He also finds the nomenclature of the elements of the 
mandible to be erroneous. This resulted from a miscohception by the 
artist of the lettering on my original drawing, which I find to be correct, 
and which in the present memoir is correctly copied. In the same way the 
small ‘‘ supersquamosal” will be found described in the present paper. 
The question as to the determination of the bones forming the roof of" 
. the cranium receives new light from Dr. Seeley’s remarks. This has been 
much needed by American naturalists, for I have been unable to find in 
the whole range of the literature of the subject an English description of the 
osteology of the head of Ichthyosaurus, which is at all complete; and the 
figures are not more instructive. Dr. Seeley’s statement, that the flat 
bone on the inner side of the temporal fossa, continuous in our specimen 
with the squamosal, is usually separated from the latter by suture, is valua- 
ble, and suggests that the element may be parietal and not homologous with 
the similar plate in Dicynodonts. This possibility has existed in my mind 
all along, but what are thus probably sutures in two of our specimens have 
looked as much like fractures. As to the bones suspected to be nasals, I 
find that of the left side present in a specimen of J. intermedius, besides 
that from Barrow, but wanting in one of J. tenuirostris. As observed by 
Seeley, the absence of a bone in a fossil has little weight in evidence of its 
* Origin of Genera, 1868; Hypothesis of Evolution, 1870. 
