Observations on Trilobites. 183 



One of the specimens of I. gigas was accidentally fractured 

 in such a manner, as to exhibit the under side of (he animal. 

 PI. XII. fig. 2. The appearances presented by this fortunate 

 fracture, induced many to believe that they were true teeth : 

 upon the supposition that these Trilobites resemble most the 

 Limulus, we endeavoured to discover something analogous to 

 them in the latter animal. Our inquiries were crowned with 

 success. It may be necessary to premise, that in the Limulus 

 the anterior part of the buckler is reflected beneath the animal, 

 and forms a flat horizontal surface. This surface is bounded 

 by two lines, which are the terminations of curved planes 

 arising from the superior internal part of the buckler. These 

 three planes meet and form at their point of junction, an acute 

 hollow spine nearly over the mouth, and directed backwards. 

 This spine is more evident in young than in old individuals, 

 and its posterior angle, as we look towards the inferior side of 

 the animal, is beset with bristles, resembling in arrangement 

 and direction those on the first articulations of the feet. In 

 the Isotelus, this spine becomes double ; they are broader 

 and curved towards each other. They are six lines in length, 

 the terminations sharp and translucent. The angles in front 

 of the observer, will of course be recognised as the junction of 

 the descending planes of the internal crust of the buckler. 

 PI. XII. Fig. 2 represents these processes, and fig. 3. analo- 

 gous parts of the Limulus. It will be understood, that in the 

 latter figure we look at the finder surface of the buckler, while 

 fig. 2. exhibits the upper side of this surface, the buckler 

 being removed. This last was selected in order to show their 

 triangular structure, and it would seem that the specimen from 

 which it was taken, was probably the cast-off shell of a Trilo- 

 bite ; at least the Limulus creeps out of its shell through a 

 similar separation of the whole anterior part of the buckler. 

 It is not to be considered at all, as the mouth of the animal, 

 which is probably placed between these lunated processes. 



