On Cyrtoma, a new genus of Fossil Echinida. 1 73 



with the peculiar form of the skeleton in which a tendency 

 to advance from the radiated structure to that of the higher 

 animals is evinced, having indicated their place at the head of 

 the Echinodermata, it is extremely probable that the depressed 

 pentangular Clypeasters will be found to connect the series 

 of affinities with the globular Echini by means of the Scutellae, 

 in which we find the altitude of the body gradually increase 

 from these to the new genus Cyrtoma; but as the majority of 

 the species of all these intermediate groups are extinct, 

 their natural arrangement becomes a matter of the greatest 

 difficulty, and can only be attempted with any chance of suc- 

 cess after the animals of existing species shall have been more 

 known than they are at present. From the oval Echinus 

 we may trace a connection to the elongated Spatangus, and 

 from this form to the Holothuria. It is singular the lit- 

 tle regard that has been paid to the natural affinities of 

 the Echinodermata, considering the rank of those naturalists 

 who have devoted themselves to their study. Thus M. 

 Agassiz and others place the Holothuria at the head of the 

 Echinodermata, and the Asterias at the lower end of the 

 order, and Scutella next to Cidaris and the true Echini. 



M. de Blainville, who professes to be guided by the general 

 form and structure of Echinidae, arranges them nevertheless 

 according to the situation and character of the mouth, and 

 introduces ten genera between the Galerites and the Anan- 

 chites, although the forms of these latter groups differ but 

 slightly from each other. 



It is in the form of the disc that all these animals destitute 

 of moveable extremities or limbs afford the most constant and 

 significant characters. In the Asteridse we see the disc 

 branched out into five equal points ; in the Rotulse the body 

 is depressed, but the radiated points are small, and confined 

 to one part of the disc ; in the Scutellae no projecting arms or 

 radii are at all observable ; but like the Rotulse and Asteri- 

 das the body is flat, and the upper and lower surfaces 

 are distinctly separated from each other by a sharp, thin 



