422 



HARRIS HAWTHORXE WILDER OX 



Part 



iNNfR ASPfCT 



.-X V£NT. 



OuTer aspect 



Fig. 21. Two views of riglil spleniale. X 3. Coutact .sur- 

 faces with otlier bones are designated by an x. P. ant., pioce.ssus 

 anterior. 



culare," thus producing a confusion of terms. The spleuiale is a thin scale of oval shape, 

 presenting a slightly concave inner, and a slightly convex outer surface. Along the side 

 of the former is attached a row of teeth, and the outer surface shows a longitudinal divi- 

 sion into an area covered by the den- 

 tale and a free area, wliicli contrihutes 

 a little to the formation of the outer 

 surface of the mandible. 



The oval shape and lack of detail 

 of this bone render its orientation dif- 

 ficult, but its outer and inner surfaces 

 are very evident, nud in most cases 

 the anterior end is pnjlonged into a 

 point, while the postericn- end is rounded. In its natural position it rests by its lower mar- 

 gin upou the upper edge of the angulare just antei'ior to the corouoid process, and the 

 ventral half of its outer surface is closely applied to the dentale. 



The hyohranchial a])j)aratu>i. — This consists of a series of sixteen pieces, all but 

 one being entirely cartilaginou.s, and representing four of the original visceral arches. 

 Their arrangement is so easily seen from plate 65, figure 12, that a verbal description of 

 the parts would seem almost superfluous. The system is seen to contain two median 

 pieces, universally designated as the first and second basibranchials, and referred to the 

 first and second branchial arches respectively, of which they form the middle pieces or 

 copulae. Of these, the second is ossified in the adult and its free posterior end usually 

 terminates in a rounded extremity, although an occasional indix idual shows a division at 

 the end into a two or three forked form, such as occurs normally in Siren. The liyoid 

 arcli, which is attached to the anterior end of the first basibranchial, consists of two pieces, 

 an inner hypohyal and an outer ceratohyal. The first branchial arch is the best devel- 

 oped of all the branchial arches, consisting of two nearly equal pieces, cerato- and 

 epi-branchiale 1, the first of which is directly connected with both the first and second 

 basibranchial and with its opposite. The second Ijranchial arch consists of a smaller 

 epibranchial, and a rudimentary ceratobranchial, reduced to a nodule of cartilage lying 

 upon the inner side of the tUstal end of the first ceratobranchial. An epibranchial smaller 

 than the previous one is the only remnant of the third branchial arch. 



In hfe the distal ends of tiie three epibranchials support the external integumcntal 

 branchiae and furnish attachment for some of their muscles, a circumstance which has 

 often misled investigators as to the true homology of these ]iurely integumental organs, 

 since the location suggests a definite phylogenetic relation to the internal gill system. 

 In the intervals between these epibranchials there occur, even in the adult Xecturus, two 



