42 A, Agassiz— Connection between the Cretaceous 
do more shir to conic the lines of affinities, the pet 
threads of which can trace in characteristics of 
which at any ‘spec epoch seem to have little or no nteacbaral 
affinity. Let us take, for instance, the genera characteristic of 
the Chalk, anid attempt to trace their connection both back- 
ward and forward in time. 
aking these genera in their most extended signification, and 
more especially those characteristic of the Lower Cretaceous 
formations, Cidaris, Orthocidaris, Pauibcecintivs Tetracidaris, 
Goniopygus, Codiopsis, Magnosia, Cyphosoma, Pseudocidaris, 
Orthopsis, Pedinopsis, Codechinus, Stomechinus, Acrosalenia, 
Echinothuria, Pygaster, Discoidea, Holec eclypus, Pyri ina, Clypeopy- 
gus, Pygurus, Metaporhinus, Holaster, Toxaster, and comparing 
them in the first place with the genera of the Lias as far as 
they are known, we find that, with the exception of Cidaris 
-and Hypodtiadema, the forerunners of the Cidaridse and Dia- 
dematide, not a single form of the Kchinide is represented. 
To attempt to aeplaie their elaaiotslelp to the earlier types, we 
may say, in a very general way, that the Perischoechinide early 
show, on the one side, a tendency to limit the number of the 
rows of interambulacrat plates; and, on the other side, a de- 
ambulacral plates into numerous irregular rows; they are thus 
the only at — directly to such types as Oidaris on the 
one side, and to the Echinothuride on the other, the genera 
retaining at the same time the proportionally incre ambula- 
cral areas of some of the types. om the time of the Trias, 
the Cidaridz have been a most puisiatens ania and the changes 
the members of the family have passed through are restricted 
to very narrow limits, with the exception of the aberrant genera 
Heterocidaris, Tetracidaris and Diplocidaris, which retain more 
or less Paleechinoid characters while taking on a more modern 
facies 
The relationship of the a devaee edna to the Palewechinide 
I have already insisted upon where, and their affinities to 
the recent Diadematides are most close. The eo of 
day, is hs cien ch near not to nee any further elucidation. 
On the other hand, the development of the Echinide is some- 
