Description of a New Species of Apus, by John Le Conte, F. 

 L. S., &c. Read December 8th, 1845. 



Among the many valuable objects of Natural History which my 

 son procured during his late journey to the Rocky Mountains, one 

 of the most interesting is the small crustaceous animal, which is 

 the subject of the present communication. 



The genus Apus, originally considered by Linne, along with 

 the Limulus or Horse-shoe Crab of our own sea coast, as species 

 of Monoculus, received its present name from the illustrious John 

 Anthony Scopoli. To both the genera Monoculus and Limulus, it 

 bears a striking external resemblance, particularly to the latter : 

 indeed, if we consider its facies alone (setting aside the naked tail,) 

 we should be very apt to conclude that it was a close congener of 

 them. But when properly studied, there will be found a vast in- 

 terval between these animals, both as regards the structure of the 

 body, and the detail of the oral and masticatory organs. This in- 

 terval is undoubtedly filled up by numerous beings, either not 

 known or not yet sufficiently examined ; " Natura enim non facit 

 saltus :" Savigny has observed that there is as much difference be- 

 tween them as between a crab and a spider (phalangium.) 



But three species of Apus are known to Naturalists; the A 

 cancriformis the A. productus of Leach, by him called^ Lepidu- 

 rus, and the A. Montagui, of the same author. The first and last 

 of these are readily distinguished from our species by the shortness 

 of the caudal extremity, and the other by having an oval horizontal 

 lamina extending from the emai'gination of the last joint of the 

 tail* 



It may not be amiss to observe here, that the animal described 

 by Mr. Say, in the Journal of the Academy of Natural Sciences 

 of Philadelphia, vol. 1, page 437, and considered by Dr. Dekay, 

 in the fifth volume of the Natural History of this State, as an 

 Apus, cannot belong to the same genus as this which we are now 

 considering, as it was found parasitic on a crab, and has but two 

 eyes ; from the very imperfect description, it is impossible to say 

 what it is. It seems to have some relation to Caligus, but as I ob- 



* In the first volume of Major Long's Expedition to the Rocky Mountains, mention is made 

 of a species of Apus, three-tenths of an inch in length, but no detailed description is given : it 

 may belong to some other genus of Crustacea. 



