162 Report of the State Geologist. 



It frequently happens that an analogous process induces modi- 

 fications in the same direction in very distinct animals. For 

 example, one of the flying Eeptiles of the Upper Cretaceous, 

 Pteranodon^ is toothless and has a sharp beak, which probably 

 was covered with horn. If Cuvier had seen this hoad he 

 would, without doubt, have considered it that of a bird ; and he 

 would, on the other hand, have assigned the two toothed birds 

 of the same deposit to the Reptilia. The disappearance of teeth 

 and the presence of a beak are then characters which have 

 affected in the same manner very different types, Pterosaurians 

 and Birds, both adapted to the same mode of life. 



Amoug the primitive Batrachians of the group of Stegocephala, 

 we find the first tendency of the four-footed type to elongate the 

 body, multiply the number of vertebrae, diminish or lose limbs, 

 to assume, in a word, the aspect of Serpents {Dolichosoma). But 

 the serpentiform types appear in very different groups. Among 

 living animals, true Batrachians (Cecilians), animals which have 

 throughout the anatomical characteristics of the Lacertilians 

 {Amphisbmnce)^ also assume the same vermiform appearance. 

 There existed in the Cretaceous epoch, among the Lacertilians, 

 gigantic swimmers possessing more than a hundred and thirty 

 vertebrae, and with very small limbs, thus evincing a tendency in 

 the same direction. The Ophidians also form a branch of the 

 Lacertilians, in which modification has affected the external 

 organs. 



An instance often cited is the profound analogy in limbs 

 transformed into swimming expansions almost identical among 

 Reptiles such as Ichthyosaurus and Plesiosaurus, and the Mam- 

 malia such as (.^etacea. 



The invertebrates furnish numerous examples of convergence. 

 Among the Ammonites, for instance, the shell often presents a 

 considerable difference both in form and ornamentation between 

 the first volutions of the spiral, and those which appear later 

 when the animal has reached a considerable size. Bat frequently 

 the differential characters of species, genera, and even of families, 

 disappear when the animal attains its full size, so that sometimes 

 it is no longer possible to determine by extiernal appearance, for 

 the Ammonites of the Cretaceous for example, to what group the 

 animal belongs, without breaking ' the shell and examining the 

 internal volutions. We shall see what bearing this fact has on 

 the establishing of the genealogical tree of the Ammonites. 



Among the Gasteropoda the form of the shell usually cor- 

 responds as a whole with the exterior form of the body. But we 

 must beware of drawing any conclusions from the variations of 

 the form of the shell as to the variations of the internal organs. 

 A classification founded on the shell would bring together the 

 most heterogeneous types. Still variations of the shell are 

 produced only in very few directions, and the modifications 

 follow the same law in groups anatomically farthest removed 



