271 



their having been filled by shells of the genus planorbis ; 

 with a nearl}^ perfect shell, whose mouth points it out as a 

 species of the genus melania, and whose size and finely 

 rugated surface give it a close resemblance to M. fragilis of 

 Lamarck. In this specimen, also, I think the cfpening of the 

 melania is apparent. I trust that the farther investigations 

 of that gentleman will determine the existence of fresh-water 

 formations in the West Indies.* 



Insects. — The delicacy of the structure, and, indeed, the 

 nature of the substance, which enters into the formation of 

 the animals which are generally comprized under this term, 

 render them so unfit to sustain those changes which would 

 be necessary for the conservation or mineralization of their 

 remains, that very few become the subjects of our exami- 

 nation. The crustaceous coverings of the bodies and limbs, 

 and the hard wing-cases of some of the genera, are the chief, 

 and, perhaps, the only fossils which can be properly referred 

 to this class. 



The trilobites, or Dudley fossil, the living analogue of 

 which is at present unknown, and the original nature of 

 which is so little understood, that it has, by some, been con- 

 sidered as a crustaceous insect, and, by others, as a bivalve 

 shell, and has hence been designated, by Linnaeus, as ento- 

 molithus paradoxus ; by Dacosta, 2is pediculus marinus ; by 

 Hermann, as pectunculus trilobus imhricatus ; will demand 

 the student's attentive investigation. 



Its superior covering, (PI. x. fig. 1.) which is the only 

 part with which we are acquainted, is oblong, ovate, convex, 

 and marginated; the anterior wider part is gibbous, and 

 furnished with two semilunar tubercular projections re- 

 sembling palpebrae ; and posterior to, and on the inner side 



* For further illustration of this subject, the reader is re- 

 ferred to Mineral Conchology, No. lix. 



