868 The American Naturalist. [October, 
It is not necessary to go into a discussion of the details of 
internal structures and their relations to the impressed zone 
in this abstract, but it is essential to give a general -e 
of the morphogeny of the order of Nautiloids. 
This group of chambered cephalopods contains ihe. follow- 
ing classes of forms: first, straight, conical shells, type Ortho- 
ceras, pl. XVIII, fig. 1; second, curved cones, Cyrtoceras, pl. 
XVIII, fig. 2; third, loosely coiled, open whorled cones, Do., 
fig. 3; fourth, coiled cones with the whorls more or less envelop- 
ing, Do., fig. 5. The fourth and fifth forms are usually 
included in the old genus, Nautilus. Practically, it is better 
to designate the first class as orthoceran, the second as cyrto- 
ceran, the third as gyroceran, and the fourth and fifth as nau- 
tilian. In tracing genetic series through time they are found 
to diverge in their evolution, starting with the orthoceran and 
passing through parallel lines of forms, many of the genetic 
series having in succession cyrtoceran, gyroceran and even nau- 
tilian forms of the fourth and fifth classes. Others are not so 
perfectly parallel, stopping short with the cyrtoceran class of 
forms or the gyroceran. Many also begin with cyrtoceran 
shells, while others diverge from the gyroceran, and still other 
series have only nautilian shells of different grades of close 
coiling and involution. 
The applieation of the law of repetition in heredity to the 
chambered shell-covered Cephalopods, shows that the straight 
orthoceran shells, pl. XVIII, fig. 1, were repeated in the young 
of the curved cyrtoceran forms, pl. XVIII, fig. 2, and these forms 
in their turn in the young of the gyroceran forms, pl. Do., fig. 3 ; 
and this may be seen by comparing the young or apical part 
of eaeh shell represented in outline with the full grown shells 
of the preceding figures. The apex of fig. 2, with the whole 
of fig. 1; the apex of fig. 3, with the whole of fig. 2. It will 
be understood, of course, that the figures in outline represent 
full grown shells, except when otherwise explained and that 
they were built like the shells of figs. 1-2, by an animal liv- 
ing in their interiors and adding band after band of shelly 
matter to the exterior, but in these outlines the shell is sup- 
