242 ESSAY ON CLASSIFICATION. 



Crinoids, Asterioicls, Echinoids, and Holothurioids would 

 be conceded also as such natural orders; among the Aca- 

 lephs the Beroids, and perhaps also the Discophorse and 

 Hydroids; while, among the Polyps, the Haley onoids consti- 

 tute a verynatural orderwhen compared with the Actinoids. 



Let us now consider these orders with reference to the 

 characteristic forms they include. The forms of true 

 Testudo, of Trionyx, and of Chelonia are very different 

 one from the other; and yet few orders are so well cir- 

 cumscribed as that of Chelonians. The whole class of 

 Fishes scarcely exhibits greater differences than those 

 observed in the forms of the common Sharks, the Saw- 

 fishes, the common Skates, and the Torpedo, not to speak 

 of the Cyclostomes and Myxinoids, if these families were 

 also considered as members of the order of Placoids. The 

 Ganoids cannot be circumscribed within narrower limits 

 than those assigned to them by J. Miiller; and yet this 

 order, thus limited, contains forms as heterogeneous as 

 the Sturgeons, the Lepidosteus, the Polypterus, the Amia, 

 and a host of extinct genera and families, not to speak of 

 those families I have associated with them, and which 

 Prof. Miiller would have removed, which, if included 

 among the Ganoids, would add still more heteromorphous 

 elements to this order. Among the Decapods we need only 

 remember the Lobsters and Crabs to be convinced that it 

 is not similarity of form which holds them so closely 

 together as a natural order. How heterogeneous the 

 Bryozoa, the Brachiopods, and the Tunicata are among 

 themselves, as far as their form is concerned, everybody 

 knows who has paid the least attention to these animals. 



Unless, then, form be too vague an element to charac- 

 terize any kind of natural groups in the animal kingdom, 

 it must constitute a prominent feature of families. I 



