Geology and Natural History. 489 
centration of development.” He speaks of the sudden appearance 
of distinct types in Paleozoic and later time as an “acknowl- 
_ edged” fact; of distinct types as far most numerously evolved in 
Paleozoic type than later; of the field of variation as decidedly 
narrower in the Mesozoic than in the Paleozoic; observes that the 
Separation of the grander groups under the general type took 
place rapidly in the Paleozoic ; and states as deduced principles 
that “types are evolved more quickly and exhibit greater struc- 
tural differences between genetic groups of the same stock while 
still near the point of origin than they do subsequently ;” that 
Mr. Hyatt makes no reference to the vastly greater length of 
Paleozoic than Mesozoic time— robably not less than five times 
longer, if not ten—which fact bears on the value of such. deduc- 
_ The uncoiled forms which occur in the later part of Mesozoie 
time are recognized as degraded or retrogressive forms, the em- 
bryonic forms in all these uncoiled kinds being coiled; and the 
return to the earlier Paleozoic forms is made part of the proof as 
to the derivation of the group from those early species. 
Tn connection, he makes the following reasonable statement : 
“Slaves of the embryological lamp consider that they must 
associate all forms which have similar embryos, and dissociate in 
classification all forms having different embryos. As a matter of 
experience the surest guides of affinity are the adult gradations 
of forms. These show that the Nautiloidea and Ammonoidea, 
with comparatively distinct embryos, are nevertheless more closely — 
related than the Belemnoidea and Ammonoidea which have pre- 
cisely similar embryos, and that Sepioidea and Belemnoidea, 
ze have very distinct embryos, must also be closely affili- 
ated, 
2. The Geological History of Serpentines, including Studies of — 
L're- Cambrian Rocks ; by T. Srprry Hunt. Trans. Roy. Soe. 
Canada, i, section 4. 1883. Montreal.—Dr, Hunt, reviewing’ a 
the subject are overlooked. No sufficient use is made of the 
Am, Jour. Sct.—Turep Series, Vou. XXVII, No. 162.—Junz, 1884. ; 
33 
