362 Prof. A. Agassiz on Palceontological 



Although we know nothing of the embryology of the 

 Salenidse, yet, like the Cidaridas, they have in a great mea- 

 sure remained a persistent type, the modifications of the 

 group being all in the same direction as those noticed in 

 the other Desmosticha — a greater number of coronal plates, 

 the development of secondaries and miliaries, combined with 

 a specialization of the actinal system not found in the 

 Cidaridse. 



An examination of the succession of the Echinoconidaa 

 shows but little modification from the earliest types; the 

 changes, however, are similar to those undergone by the 

 Clypeastroids and Petalosticha, though they do not extend to 

 modifications of the poriferous zone, but are mainly changes 

 in the actinostome and in the tuberculation. In fact, the 

 group of Echinoconidse seems to hold somewhat the same 

 relation to the Clypeastroids which the Salenidse hold to the 

 Cidaridge ; and the earliest genus of the group {Pyg aster) 

 has remained, like Cidaris, a persistent type to the present 

 day. 



The earliest Clypeastroids are all forms which resemble 

 the Fibularinse and the genera following Ecliinocyamus and 

 Fibularia ; they are mainly characterized by the same changes 

 which an Echinarachnius or a Mellita, for instance, under- 

 goes as it passes from its Ecliinocyamus stage to the Laganum 

 or Encope stage. The comparison is somewhat more compli- 

 cated when we come to the Spatangoids. The comparison of 

 the succession of genera in the different families, as traced in 

 the Desmosticha and Clypeastroids, is made difficult from the 

 persistency of the types preceding the Echinoneidaj and the 

 Ananchytidse, which have remained without important modi- 

 fications from the time of the Lower Cretaceous ; previous to 

 that time the modifications of the Cassidulidae are found to 

 agree with the changes which have been observed in the 

 growth of Echinolampas. The early genera, like Pygurus, 

 have many of the characteristics of the test of the young 

 Echinolampas. The development of prominent bourrelets 

 and of the floscelle and petals goes on side by side with that 

 of genera in which the modification of the actinostome, of 

 the test, and of the petals is far less rapid, one group re- 

 taining the Echinoneus features, the other culminating in the 

 Echi?wlampas of the present day, and having likewise a per- 

 sistent type, Echinobrissus, which has remained with its main 

 structural features unchanged from the Jura to the present 

 day. That is, we find genera of the Cassidulidse which recall 

 the early Echinoneus stage of Echinolampas^ next the Cara- 

 tomus stage, after which the floscelle, bourrelets, and petals 



