354 



On two American Species of the Genus Hydrachna. 



hours each, and in proof of it appeal to the supposed order of veg- 

 etable and animal relics in the rocky strata. Cryptogamous plants 

 and the lowest orders of animals, say they, are found in the transition 

 and oldest secondary rocks, but the bones of quadrupeds are found 

 only near the surface in the last deposits ; and this they maintain is 

 agreeable to the order of Moses, and the fact is cited as furnishing a 

 striking coincidence between scripture and science — between reve- 

 lation and modern discoveries. But can there be much in this — is it 

 possible that twenty four hours' difference in the ages of two animals 

 should make such a wide difference in the disposition of their re- 

 mains — that the reptile which is only a few hours older than the 

 beasts of the field, the remains of the first should be entombed thou- 

 sands of feet below the last — that cryptogamous plants only a few 



days older than the coniferae, should be found so much deeper in the 

 bowels of the earth ? 



Art. X. — On two American Species of the Genus Hydrachna ; 



by James D. Dana and James Whelpley. 



Read before the Yale Nat. Hist. Soc, May 5, 1836. 



The Hydrachnae, or water spiders, which are here described, 

 were found as parasites on several species of Unionidae (fresh water 

 clams) collected in the canal near this city.* They usually occur 

 on the branchial leaflets within the mantle of these animals, where 

 they live, extracting their nutriment by means of a sucker-like 

 mouth. * 



v 



The examinations of the species of this genus, seem as yet, to 

 have been hardly sufficient in number or accuracy for an exact de- 

 termination of generic characters. In the following description, 

 it is therefore thought advisable, to state first the characters which 

 these species possess in common, without reference to the generic 

 distinctions heretofore proposed ; and afterwards, their distinguish- 

 ing peculiarities. 





The body of these animals is enveloped in a coriaceous mem- 

 brane extending undivided over the head, thorax and abdomen. 



* New Haven. 



