182 On Cyrloma, a new genus of Fossil Eehinida. 



in the general conformation of the skeleton, but the parts of 

 which the star-like figure is composed, evinces a tendency to 

 pass from the radiated, to the annulose structure. 



2. Such groups of Eehinida as present a distinct rosette, 

 and central mouth, with the ambulacral and other series in- 

 terrupted at the disc, follow next after Asterias, connecting 

 that group with Echinus. 



3. That in Echinus every part of the skeleton presents 

 aspheroidal structure, and the series of plates of which it 

 is composed continue to grow as long as the animal lives, by 

 means of layers of new deposit on their margins. That in 

 some species the plates all become united, so as to form a 

 thick unyielding shell, in which certain vertical fissures form 

 as life advances to the close, and the skeleton separates into 

 five sections like the ribs of a melon. 



4. To the Echini and Cidarites, the Ananchites, Spatangi, 

 and other groups in which there is no distinct rosette on the 

 lower surface of the body, next succeed. 



5. That some of these last forms become elongated on 

 that part of the disc which lies between the two posterior 

 ambulacra, and approximate to the Holothuria ; while others 

 become flat, and the disc either nodular or serrated as in 

 Rotula, and thus evince a tendency to return once more to 

 the branching depressed form of Asterias. 



Having endeavoured to trace the affinities of Echino- 

 dermata, it will be seen that the interval between the 

 starfish and Echinus is wider than has hitherto been sup- 

 posed, and that the known groups which occupy this space 

 are few compared to those which succeed the genus Echinus. 

 There are however some, particularly of the Scutellcs, for 

 distinct reasons already stated, that may be assigned to it ; 

 our new group Cyrtoma is evidently nearly allied to these, 

 and must assist in rendering the circle of affinities in this 

 place more perfect. The Scutellce and Cyrtomce, together 

 with such other forms as may be found to possess a distinct 

 petaloid star on the lower, as well as the upper surface of 



