178 Observations on Trilobites, 



Breadlh of the middle lobes 1*6 

 of the lateral lobes *8 



Depth of the buckler *9 



of the lateral lobes at 

 their free extremities *6 



These dimensions exhibit an animal of large size, but the 

 Cabinet of the Lyceum possesses a very considerable portion 

 of an individual of this species, which, when extended, must 

 have been at least seventeen inches long. M. Alexandre 

 Brongniart, speaks of an Ogygia 39 centimetres, or about 

 fifteen ^ind a half English inches injength. The thickness of the 

 animal may be determined by those specimens, PI. XII. fig. 1. 

 in which the head and tail are closely applied to each other. 

 The form and position of this latter organ, it may be cursori- 

 ly remarked, bears a striking resemblance to that of the Hippa 

 emerita. 



Isotelus planus.* Plate XIII. fig. 2. 

 Head more rounded than the preceding, less elevated. Tail 

 flat, rounded. 



Total length 2-1 



Breadth 1 • 1 



Length of head '6 



of abdomen '8 



of tail -7 



Cabinet of the Lyceum. 



This may possibly prove the young of the preceding species. 

 The relative proportions of its buckler and tail vary considera- 

 bly from those of the J. gigas, as may be seen by the dimen- 

 sions given above. The depth of the lateral lobes, which ex- 

 ceeds three tenths of an inch, would almost of itself determine 

 it to be a new species. Should it, however, be considered 

 identical with J. gigas, it furnishes additional proof of the 

 very rapid growth of these animals. 



Hitherto these are the only species of the genus yet discov- 

 ered. The Asaphus cornigerus (Schlottheim) may perhaps bo 





