EVOLUTION OF FOSSIL CEPHALOPODA. 237 
species is known more accurately than that of most liv- 
ing forms. 
Vertebrates are out of the question for this sort of 
work, being too highly accelerated in their development ; 
the stages that might be useful in phylogenetic study 
are gone through before the animal is capable of being 
preserved as a fossil. In the ccelenterates the relations 
between Cenozoic and Paleozoic forms are not under- 
stood, and the ontogeny of the group does not show 
changes that are striking enough to throw much light on 
their history. In the #chinodermata poor preservation 
of fossil forms, especially of the young, makes ontogen- 
etic study very difficult, but Dr. R. T. Jackson * has 
been able, by a study of development of plates of the 
sea urchins, to throw some light on the phylogeny of 
the group. The only crinoid of which the development 
is known is Antedon, which, unlike all the others, is free- 
swimming when adult, although attached by a jointed 
stem asa larva. The investigations of Wyville Thom- 
son, Bury, and others, show that the embryo is like the 
larva of certain annelid worms; after a short free stage 
this embryo settles down, attaches itself to some object, 
begins to secrete a stem and jointed calyx, or body cup. 
In this stage it is like the /chthyocrinoidea of the Paleo- 
zoic. Thencertain of the plates are resorbed, the char- 
acter of the jointed stem changed, and the animal is like 
Pentacrinus of the Mesozoic and Cenozoic. At the end 
of its larval period Antedon frees itself from its fixed 
position, loses all resemblance to Pentacrinus, and for 
the rest of its life is a free-swimming pelagic form. 
The most satisfactory groups for this work are the 
Brachiopoda, the Mollusca, and the Crustacea, 
* Studies of Paleechinoidea. Bull. Geol. Soc. Amer., vol. vii, 
pp. 71-254. 
