200 The American Geologist. ^P"^' ^^°"^- 
the animal on the shell or whether the chambered portion of 
the shell parted from the protoconch. Both cases are possible, 
the former more probable. I have seen its oval impression on 
the .succeeding volution of a Nautilus barrandd from the al- 
pine Keuper. The first chamber of the Nautiloidea is hence 
not the protoconch, but the first air-chamber, which at the low- 
er end shows a scar for the passage of the siphuncle from the 
protoconch into the chambered portion of the shell like that 
in the orthoceratites. 
"6. The semi-involute Nautiloidea, the cyrtoceratites in a 
broad sense, are not transitional types from the orthoceratites 
to the involute Nautiloidea, but regressive types of the latter. 
The involution in every case presupposes a free state of the 
individual, hence at least an early detachment if not an absence 
of attachment. 
"7. The forms with contracted ostium or aperture, as 
Gomphocrras, Phracrnwreras, Tetrameroceras, H exameroceras , 
were probably imbedded in the silt with their entire shell and 
protruded only their arms and their funnel, which here as in 
other mollusks was a sipho, i.e. a breathing tube. 
"8. The ammonites and belemnites were free from their 
inception, as they have retained their embryo-chamber in form 
of a calcareous ovoid bulb on their chambered shells. 
"9. The rostrum of the belemnites was not a rostrum, i e. 
a water-cutter, but a paxillus, a post to be driven into the 
ground ; the belemnites were hence not free and, as generally 
supposed, rapidly swimming, but were sedentary animals. 
"10. The higher "Dibranchiates," the ink-fishes proper, 
have become crawlers or have acquired a retrograde swim- 
ming motion by means of the funnel. With the exception of 
the cuttle-fishes, which developed from the belemnites and 
which, in Spirulirostra and Spirula, show interesting regres- 
sive characters, the skeleton of the remaining dibranchiates, is 
reduced either completely in the benthonic, essentially creeping 
octopods, or has, after complete obliteration of the "rostrum," 
become a flexible axial support, which consists of conchiolin 
and is comparable to the vertebral column, in the slender 
Oigopsidse which became better swimmers and possess even 
paired terminal fins. 
