Dall — Hinge of Pelecypods and its Development. 445 



each. Omitting mention of the Permian, or uppermost por- 

 tion of the Carboniferous system, which seems to be absent 

 here, the hiatus between the Lower Cretaceous, and the Car- 

 boniferous strata of the Climate Mountains amounts to at least 

 the whole of the Jura and Trias. 



This hiatus is no greater than is exhibited in others of the 

 mountain uplifts in the region under discussion, and not so 

 great as it is in some cases. Indeed, so far as I am now aware, ' 

 the hiatus hetween the Lower Cretaceous and the next under- 

 lying rocks is nowhere in all that region less than it is at the 

 locality in the Chinate Mountains just mentioned. That is, 

 both the Jura and Trias are believed to be absent there. That 

 neither the Jura, Trias, or Lower Cretaceous occur between 

 the Upper Cretaceous and Carboniferous in central New Mex- 

 ico, accords with observations that I made there during the 

 past season. So far as both the Jura and Trias are concerned, 

 I have not yet been able to obtain any satisfactory proof that 

 either of these geological divisions are represented by any 

 North American strata south of the 34th parallel of latitude.* 



Art. LY. — On the Hinge of Pelecypods and its Develop- 

 ment, with an attempt toward a better subdivision of the 

 group • by Wm. H. Dall, Paleontologist, U. S. Geol. Sur- 

 vey, and Curator Dept. of Mollusks, U. S. National Museum. 



The attempt to divide the class Pelecyjpoda or Lameilibran- 

 chiata into orders has so far been unsuccessful, or at least the 

 subdivisions adopted have from time to time been found unsat- 

 isfactory, on account of the discovery of forms which combine 

 in their organization characters which had previously been 

 regarded as diagnostic of important subdivisions, such as orders. 



This has resulted from the selection of characters as diagnos- 

 tic which are really not fundamental in the evolutionary his- 

 tory of the minor groups. As we gradually become acquainted 

 with the mutability of the adductor muscles, the gills, the 

 arrangements for retracting the siphons and other factors in 

 the mechanics of these organisms, the classification based upon 

 their mutations has gradually ceased to satisfy students though 

 one phase or another of it may still retain a place in ordinary 

 text books. 



* Reference is not here made fco the " Dinosaur Sands " which lie at the base 

 of the Comanche Cretaceous upon both sides of the Paleozoic area in Texas. 

 These beds I have provisionally included in the Lower Cretaceous, but it is prob- 

 able that they represent the E J otomac formation of the Atlantic coast, region, and 

 it is regarded as possible that they represent the uppermost Jurassic of Europe. 



