Agassiz— Paleontological and Embryological Development. 377 
surmounted by few and large primary tubercles, supporting 
proportionally equally large primary radioles, simple rectilinear 
poriferous zones, no petaloid ambulacra,—in fact, scarcely one 
of the features we are accustomed to associate with the Clype- 
astroids is as yet prominently developed. But rapidly, with 
increasing size, the number of primary tubercles increases, the 
* Among the Clypeastroids I have examined the young of Echinocyamus, Fib- 
ularia, Mellita, Laganum, Echinarachnius, Encope, Clypeaster, and Echinanthus. 
