On Cyrtoma, a neiv genus of Fossil EcMnida. 169 



increase in bulk after the number of plates have been 

 completed. Nor do we find the altitude between poles 

 of the spheroid correspond with the age and bulk of 

 Echini, as we might expect from the observations of M. 

 Agassiz; for of the five individuals I have examined, the 

 smallest has the same form as the largest, being depres- 

 sed or flattened between the poles, while another indi- 

 vidual, three inches in diameter, has an equal altitude 

 with the largest.* M. Agassiz reduces the various forms 

 of Echinodermata to three types, namely, the tubular 

 (Holothuria), spheroidal (Echini), and starfish (Asterice), 

 but these types he observes may be reduced to one, 

 in as much as the tubular form may be considered as 

 an elongated spheroid, thus uniting the Holothuria with 

 the Echini, and we have already seen how these last 

 are united to the starfish by MM. Blainville and Agassiz. 

 I confess that unless the object is to establish the above 

 as a natural series of affinities, I cannot very clearly see 

 how this reduction of types can simplify the subject; and 

 if the object was to trace the series of natural affinities, it 

 is singular that M. Agassiz though aware of the peculiar 

 relation between the starfishes and certain genera of flat 

 Echinidas, in which the upper and lower surfaces of the 

 body are distinct, should have laboured to establish a 

 direct affinity between the globular Echini and the As- 

 terics. The discovery by M. Agassiz of the bilateral 

 disposition of the parts composing the Echinodermata is 

 however a valuable addition to our knowledge, and will 

 doubtless prove an important step towards the natural 

 classification of the Radiata ; but until we are far 

 better informed regarding the habits and structure of 

 these animals, and the investigation be undertaken by na- 

 turalists possessed of a clear perception of the difference 



* Figs. 1 and 2, plate III, represent the elevation of two individuals, the one 

 four and a half inches in diameter, the other three, though the altitude of both 

 is the same. 



