878 General Notes. (September, 
We find then that the entire change from the generalized hex- 
apod to the ordinally specialized hexapod was made in the inter- 
val between the close of the Paleozoic period and the middle, we 
may say, of the Mesozoic. These significant changes were 
ushered in with the dawn of the Mesozoic period, and the Tri- 
assic rocks become naturally (together with the Silurian) the 
most important, the expectant ground of the student of palzeon- 
ology. 
Hitherto for fifty years the Carboniferous period has claimed 
this interest as its birthright. 
It would then appear that the geological history of winged 
insects, so far as we know from present indications, may be 
summed up in a very few words. Appearing in the Silurian 
period, insects @ontinued throughout Palaeozoic times as a gener- 
alized form of Heterometabola which, tor convenience, we have 
called Palzodictyoptera, and which had the front wings as well 
as the hind wings membranous. 
On the advent of Mesozoic times a great differentiation took 
place, and before its middle all of the orders, both of Hetero- 
metabola and Metabola were fully developed in all their essential 
` features as they exist to-day, the more highly organized Metabola 
at first in feeble numbers, but to-day, and even in Tertiary times, 
as the prevailing types. The Metabola have from the first 
retained the membranous character of the front wings, while in 
most of the Heterometabola, which were more closely and 
directly connected with Palzeozoic types, the front wings were, 
even in Mesozoic times, more or less completely differentiated 
from the hind wings as a sort of protective covering to the latter, 
and these became the principal organs of flight. 
Garman on Dipymopus.—Mr. Samuel Garman has published a 
description! of the shark Chlamydoselachus anguineus Garm., an 
introduced into his paper some comments on my paper? on the 
extinct shark Didymodus, which has been found in the beds of 
Permian age in Texas. Mr. Garman’s comments are in the form 
of a criticism which denies the existence of some of the lead- 
ing points of structure of the skull which I have pointed out. 
The surprise which these criticisms occasion increases when it 
is understood that they are derived “ from a study of the illustra- 
tions,” and not of the specimens themselves. And Mr. Garman 
appeals to “a comparison with the plate in the Proceedings” (of 
the American Philosophical Society) to “show whether they can 
be justified” (p. 29). To utter the sweeping conclusions reached 
by Mr. Garman on such a basis as this, is, to say the least, haz- 
ardous ; and it is a comparatively easy task to show that they are 
__ Wrong, by reference to the specimens themselves. On p. 573 of 
* Bulletin of Museum Comparative Zodlogy, Cambridge, x11, July, 1885. 
_ Proceedings American Philosophical Society, Philadelphia, 1884, p. 572+ 
