Review of liecenf GeoUxjical LiterafAive. 257 
sutures vary with age, thus making, with the external shell, three rec- 
ords of mutations. The value of such material for phylogenetic study 
can be best appreciated in comparison with that oVjtainable in other 
groups, such as the vertebrates. The author says upon this point : 
"How unreasonable it would seem to a student of fossil Mammalia, if he 
were requested to do what it would be appropriate to require from a 
student of the fossil Cephalopoda, viz., to describe from the investiga- 
tion of a single perfect fossil skeleton of an adult, not only the charac- 
teristics of the skeleton at the stage of growth at which the animal 
died, but the developmental stages of this same skeleton, and in case it 
were the remains of an old, outgrown animal, also, the retrograde meta- 
morphoses through which it had passed during its last stages of decline. 
It might require a life time to make out the stages of a single species of 
mammal satisfactorily from the isolated specimens which would be 
found and the attempt would be hopeless for all the youngest stages of 
growth, while the bones were .still cartilaginous. 
"This kind of evidence, however, is readily obtainable among fossil 
Cephalopods with relation to the shell and other hard parts as among 
living animals, and it can be obtained in good collections everywhere, 
whether 'in situ' or in museums. Thus it is possible to study the rela- 
tions of these fossil forms very minutely and with the certainty of pos- 
sessing a clue to their true relations, which is rarely obtainable even 
among existing animals. For among these we have only the embryos 
and young of contemporaneous forms and necessarily lose all relations 
of succes.sion in time, unless the investigation embraces a prolonged se- 
ries of experiments or is more or less historical, and even then the facts 
cannot have a very wide chronological range." 
The classification adopted is as follows: Class Cephalopoda, with two 
subclasses: I. Tetrabranchiata, containing the orders Nautiloidea and 
Ammonoidea: II. Dibranchiata, containing the orders Belemnoidea and 
Sepiodea. 
The four orders show a common origin through their development, 
their morphology and their having a similar emljryonic shell, the proto- 
conch. The primitive forms, the nautiloids, gave origin to the di- 
branchs through the gradual modification of the external shell into an 
internal organ. Transitional forms are met with, such as AulncoceraH 
of the Trias. The development of Loligo, as shown by Lankester, in- 
dicates an enclosure and suppression of the external shell. Such pre- 
dictions are now substantiated by a mcjrphological study covering the 
geological history of the group. 
The author shows that, from a number of old nautiloid stocks, there 
arose successively series of straight, arcuate, gyroceran and involute 
shells, and that the old idea of a gradual progression of similar forms 
through the order as a whole, the arcuate and gyroceran in later peri- 
ods and the involute last, can only be applied to single branches of the 
phylum. From a general view of the order, Barrande showed that there 
were straight, arcuate and coiled forms appearing all through Paleozoic 
time and considered that this progression of form did not indicate phy- 
