On Cyrtoma, a new genus of Fossil Echinida. 187 



equal in altitude to a third of the diameter ; anal ridge prominent ; 

 three anterior ambulacra broad lanceolate, with two rows of 

 transverse holes. Lower surface merely marked by five promi- 

 nent ambulacral ridges without pores- 



6. Cyrtoma astrobola, J. M. t. v. figs. 10, 11, 12. 



About one-sixth part of an inch in diameter ; disc oval ; mouth 

 and anus large, but the ambulacra are not developed, 



I should describe this last as a new genus, if I had the least 

 doubt as to its being the young of some one of the fore- 

 going species. Its oval disc, the situation of the mouth 

 and anus, together with the circumstance of its occurring 

 with the other forms, leave no room to question the fact 

 of its being a young individual of this group. It may indeed 

 be the young of some one of the foregoing species that had 

 but escaped from its ovum a few hours, at all events a few 

 days only before its destruction, the causes of which were 

 connected with such mighty physical changes. At all events, 

 the circumstance is one of deep interest, for we might 

 have expected that a succession of deposits would have 

 buried it much deeper in the earth than the superficial bed 

 in which its remains have been discovered. There are 

 however a great many elements to be taken into account 

 before we can come to conclusions in subjects of this na- 

 ture ; and where the materials are so abundant it would be a 

 national reproach > to leave them unexplored, when their 

 investigation might prove of so much importance in casting- 

 further light on the history of the earth. One new fact is 

 however established in zoology by the fossil embryo in 

 question, namely, that the mouth and intestinal aperture 

 of young Echinida are developed before the ambulacra ap- 

 pear ; and it is curious enough that while recent species are 

 so abundant, we should first acquire a knowledge of this 

 circumstance from one that has been so long extinct. 



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