390 MEMOIRS OF THE CARNEGIE MUSEUM. 



It must be regarded as an exceedingly fortunate circumstance for paleontology 

 that calcareous deposits of the kind known as Lithographic Stone should occur 

 in the Upper Jurassic rocks of various parts of Europe, more especially in northern 

 Bavaria and in the Department of Ain, France, because of the excellence of preser- 

 vation in this medium of the most delicate structures and impressions. 



Laid down in the shallow lagoons of coral atolls in the form of fine calcareous 

 ooze, the material itself, and the conditions under which it was deposited, lent 

 themselves to the registration with extreme fidelity of the structural features of 

 the organisms which were imbedded in it. In consequence it not infrequently 

 happens in the case of the fishes that we are acquainted not only with the entire 

 skeletal anatomy and the minute details of the hard parts, but even with soft 

 tissues, such as the muscular fibres, the air-bladder, the intestinal tract, the fin- 

 membranes, and in a few rare instances with the egg-capsules of cartilaginous 

 forms. The favorable circumstances of preservation, to which these structures 

 and others equally delicate, such as the plumage of Archoeopteryx and impressions 

 of Medusa' bear witness, have enabled us to study fossil organisms embedded in 

 hard rock with a degree of minuteness, which is scarcely possible elsewhere through- 

 out the stratigraphic column. 



A few words as to the general character of the ichthyic fauna of Solenhofen 

 may be appropriate. It is chiefly composed, as is the universal rule in strata of 

 Mesozoic age, of "ganoids," that is, of teleostomous fishes belonging to both the 

 Crossopterygian and Actinopterygian orders. A number of interesting survivals 

 of archaic types are to be observed, not the least remarkable among which belong 

 to the group of primitive sturgeons, Chondrostei, and to the characteristically 

 Mesozoic suborder known as Protospondyli. Examples are furnished under either 

 head by the genera Coccolepis and Homceolepis which are the terminal members 

 of the long-lived series to which they respectively belong, the Poloeonisddce and 

 SemionotidcB. Undina presents another illustration. 



The Dipnoan subclass, which formed so important an element of the Paleozoic 

 vertebrate fauna, is without known representatives in the strata under considera- 

 tion, or indeed anywhere in the Jurassic, the group having declined markedly in 

 the Trias, although its more generalized members continued to survive until 

 modern times. 



Coincident with the decline of Paleozoic lung-fishes is to be noted the almost 

 total extinction of Elasmobranchs toward the close of the older era, in which they 

 had been especially prolific. Nevertheless fragmentary remains of this class are 

 recognizable in rocks of Triassic age, and entire skeletons of undoubted Selachians 



