360 Prof. A. Agassiz on Palceontological 



structure from the actinostome to the apex, the primary tuber- 

 cles are large, few in number, surrounded by spines which 

 would more readily pass as the spines of Cidaridge than of 

 Spatangoids. The fascioles are either very indistinctly indi- 

 cated, or else the special lines have not as yet made their 

 appearance ; the ambulacral suckers of the anterior zone are 

 as large and prominent as those of the young stages of any 

 of the regular Echini. It is only little by little, with advan- 

 cing age, that we begin to see signs of the specialization of 

 the anterior and posterior parts of the test, that we find the 

 characteristic anal or lateral fascioles making their appear- 

 ance ; only with increasing size that the spines lose their 

 Cidaris-like appearance, that the petals begin to be formed, 

 and that the simple actinostome develops a prominent posterior 

 lip. In the genus Hemiaster the young stages are especially 

 interesting, as long before the appearance of the petals, while 

 the poriferous zone is still simple, the total separation of the 

 bivium and of the trivium of the ambulacral system, so cha- 

 racteristic of the earliest Spatangoids (the Dysasteridse), is 

 very apparent*. 



From this rapid sketch of the changes of growth in the 

 principal families of the recent Echini we can now indicate 

 the transformations of a more general character through which 

 the groups as a whole pass. 



In the first place, while still in the Pluteus stage all the young 

 Echini are remarkable for the small number of coronal plates, 

 and for the absence of any separation between the actinal and 

 abactinal systems and the test proper. They all further agree 

 in the large size of the primary spines of the test, whether 

 it 'be the young of a Cidaris 7 an Arbacia, an Echinus, a 

 Clypeaster, or a Spatangoid. They all in their youngest 

 stages have simple vertical ambulacral zones ; beyond this 

 we find, as changes characteristic of some of the Desmosticha, 

 the specialization of the actinal system from the coronal 

 plates, the formation of an anal system, the rapid increase in 

 the number of coronal plates, with a corresponding increase 

 in the number of the spines and a proportional reduction of 

 their size, the formation of an abactinal ring, and the change 

 of the simple vertical poriferous zone into one composed of 

 independent arcs. 



In the Spatangoids and Clypeastroids we find common to 

 both groups the shifting of the anal system to its definite 

 place, the modifications of the abactinal part of the simple 



* For this sketch of the emhryology of the Petalosticha I have ex- 

 amined the young of Echinolampas, Eehinoneus, Echinocardium, Brissop- 

 sis, Ayassizia, Spatangus, Brissus, and Hemiaster. 



