58 \V. K. GREGORY. 



The foregoing table summarizes the relations in various forms 

 of the first and second visceral arches to the chondrocranium, 

 and gives the descriptive terms used by authors and the terms 

 here adopted. 



Upon this analysis the following definitions may be based. 

 PAL/EOSTYLY : "First" visceral or mandibular and "second" 

 visceral or hyoidean arches retaining in large part their primi- 

 tive function as gill bearers, subequal in size, with slight or 

 no connections with the chondrocranium ; preoral gill 

 arches. Hypothetical. 



C/ENOSTYLY : "First" and "second" visceral arches modi- 

 fied in correlation with feeding, unequal in size, one or more 

 of the elements attached to the cranium. Preoral arches 

 changed in function (=labial cartilages and trabeculae cranii.) 



A. HoLOSTYLY 1 : Platoquadrate fused with chondrocranium; 



second visceral or hyoidean arch intact, non-suspensorial 

 and free from the cranium. Holocephali. 



B. AuTOSTYLY 2 : Palatoquadrate fused with chondrocranium ; 



second visceral arch broken up and non-suspensorial, 

 the hyomandibular reduced and united with the cra- 

 nium. Dipnoi, Amphibia. 



C. HYOSTYLY : Palatoquadrate articulating with chondrocra- 



nium, hyomandibular more or less suspensorial. 



a. Hyostyly proper? Second visceral arch intact, the hyo- 



mandibular and hyoid segments forming a movable 

 suspensorium for the upper and lower jaws. Most 

 sharks, Squatina. 



b. Eukyostyly* Second visceral arch broken up, the dorsal 



segment (hyomandibular) forming the sole suspen- 

 sorium, the distal regment (ceratohyal) secondarily, 

 free from all connection with the jaws, functioning 

 solely as a gill bearer. Most rays. 



1 " Holo," in allusion, either to " Holocephali," or to the fact that probably since 

 early Palaeozoic times the palatoquadrate and the cranium have formed a continuous 

 whole. 



2 The term " autostylic," hitherto applied to both Dipnoi and Holocephali is here 

 restricted to apply only to the former. 



" Hyostyly proper " because the ordinary sharks furnish the traditional type of 

 this condition. 



' Euhyostyly," as a progression upon or development of the " hyostyly proper." 



