84 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



Origin of the Opercular Series 



The beginnings of the opercular fold may be seen in the shark Chlamydoselachus, 

 where there is a prolongation of the skin on the outer border of the glU-arches. In Chimara 

 there is a true opercular flap borne on the back of the hyoid arch and supported by carti- 

 laginous streaks which have the appearance of being serially homologous with the extra- 

 branchial cartilages of the branchial arches. 



In the most primitive known actinopterans the oromandibular arch grows backward 

 over the cheek and under the circumorbital bones; likewise the hyoid arch grows backward 

 and its covering dermal fold gives rise to the opercular, subopercular, branchiostegals and 

 interopercular, as can be seen in the larval stages of Amia calva and many other fishes. 

 Tate Regan (1929, p. 313) has suggested that the interopercular appears to be the separated 

 lower end of the subopercular. It is not found as such in the oldest actinopterans but may 

 be represented by the first branchiostegal, since in the semionotids the latter element is 

 becoming oblique in position and is manifestly equivalent also to the interopercular. The 

 interopercular is always tied by ligament to the angular projection of the mandible. Be- 

 low the interopercular there is a sharp crease separating the opercular from the branchio- 

 stegal series and permitting the former to move with the jaws, the latter with the branchial 

 arches. 



Ridewood (1904^) notes that the preopercular is associated with the "supratemporal" 

 (= tabular, extrascapular, or scale-bone) and with the pterotic and the posttemporal in 

 carrying a branch of the sensory canal system. Thus these elements, along with the cir- 

 cumorbital bones, the dermosphenotics and some others, have relatively constant relations 

 with the "lateral-line" system of the head (cf. Goodrich, 1909, pp. 220-222). 



As all ichthyologists know, the relations of the numerous sliding bony plates that 

 cover the gill-chamber are also, on the whole, remarkably constant, at least in their broader 

 features, in all the hosts of the teleosts, except when by degenerative specialization one or 

 more of the standard elements may be reduced or even disappear. It is also evident that 

 there is often a close correlation between the particular form and details of all these parts 

 and the modifications of the jaws and gill apparatus that are connected with different types 

 of food and feeding and of respiration. Moreover there is an equally close correlation 

 between the head-form and body-form, the latter in turn being connected with the method 

 of locomotion (see p. 431). 



Summary: Four Chief Stages in the Origin of the Jaws, Branchial Arches and 



Opercular Elements 



The preceding tentative review of the chief stages in the origin and early evolution of 

 the visceral skeleton may be summarized. as follows: 



I. Development of supporting tissue beneath brain-chamber and surrounding gill- 

 pouches. This tissue was moulded around the septa between the gill-tubes and gill-cham- 

 bers. It also surrounded many of the blood-vessels and nerves. 



II. Origin of localized visceral arch skeleton (= oromandibular plus branchial arches) 

 by progressive fenestration of the originally continuous supporting tissue mentioned above. 

 Stages perfectly preserved in Stensio's ostracoderms and in modern Petromyzon. At the 

 beginning of this stage the organisms were slow-moving, partly bottom-living forms with 

 flattened throats and domed heads, feeding perhaps by ciliary ingestion. 



