66 IOWA GEOLOGICAL SURVEY 



hyostylic condition, on the other hand, as exemplified by Elasmo- 

 branchs, bony-scaled fishes ("Ganoids") and modern osseous 

 fishes ("Teleosts"), the same elements are loosely articulated 

 with the primordial cranium, and the upper segment of the hyoid 

 arch forms a movable suspensorium (hyomandibular). The 

 autostylic condition was carried to a still higher phase of de- 

 velopment by terrestrial amphibians, and through them trans- 

 mitted to all higher vertebrates ; whereas the hyostylic condition 

 culminates in the extreme specialization characteristic of the 

 more modern types of fishes — those expressions of vertebrate 

 life which are most completely adapted to an aquatic habitat. It 

 may be remarked parenthetically that only a few sharks, such 

 as the existing Notidanus, and perhaps the Permian Pieuracan- 

 thus (Fig. 15, page 93), display an arrangement by which both 

 the mandibular and hyoid arches have their own separate and 

 independent connections with the cranium, and the skull in that 

 condition is said to be "amphistylic".* 



The nature of the attachment of the lower jaw to the cranium 

 being regarded as of prime importance for distinguishing major 

 divisions, the next most valuable criterion for systematic pur- 

 poses is provided by the structure of the exoskeleton. This hap- 

 pens to be constructed in such wise that two groups, both of the 

 autostylic and of the hyostylic fishes can be readily distin- 

 guished. Thus, the placoid dermal calcifications, or "shagreen" 

 covering the head and body of Chimaeroids are fundamentally 

 distinct from the bony headshield and regularly disposed im- 

 bricating squamation of Dipnoans. The same distinction applies 

 with equal force in separating Elasmobranchs from Teleostomes, 

 this latter title being Owen's comprehensive designation for both 

 "Ganoids" and "Teleosts". Among Teleostomes, the rhombic 

 was probably the more primitive form of scales, this type being 

 characteristic of the geologically older scaly fishes. Structurally 

 the scales of Dipnoans are not unlike those of Teleostomes, 

 though there are minor differences in detail. The resemblance 

 in scale-structure between such widely distinct groups is without 

 special significance, however, other than as an example of evolu- 

 tionary convergence. 



* Huxley, T. H., Proc. Zool. Soc. 1876, p. 41 ct seq. 



