710 LOUIS AGASSTZ, 



any generic differences between our living 

 sponge and the fossil. The next day brought 

 us a genuine Siphonia, another genus thus far 

 only known from the Jurassic beds ; and it is 

 worth recording, that I noticed in the collec- 

 tion of Governor Rawson another sponge, — 

 brought to him by a fisherman who had 

 caught it on his line, on the coast of Barba- 

 does, — which belongs to the genus Scyphia. 

 Thus the three characteristic genera of sponges 

 from the secondary formation, till now sup- 

 posed to be extinct, are all three represented 

 in the deep waters of the West Indies. . . . 



Another family of organized beings offers a 

 similar testimony to that already alluded to. 

 If there is a type of Echinoderms character- 

 istic of a geological period, it is the genus 

 Micraster of the cretaceous formation, in its 

 original circumscription. No species of this 

 genus is known to have existed during the 

 Tertiary era, and no living species has as yet 

 been made known. You may therefore imag- 

 ine my surprise when the dredge first yielded 

 three specimens of a small species of that 

 particular group of the genus, which is most 

 extensively represented in the upper cretaceous 

 beds. 



Other examples of less importance might be 



