PROTOSPHYR^ENA. 



151 



third to the eighth baseosts (all except the foremost being well seen in PL XXXII, 

 figs. 6, 6 a) are a regular series of stout bars, expanded at their lower end at 

 right angles to the plane of the fin, and articulating at their upper end with 

 a row of six shallow pits in a flat surface of the pectoral arch. This arrangement 

 was first described by Cope, 1 but the pectoral arch itself was not correctly 

 interpreted until Hay 2 compared it with the corresponding arch in the existing 

 Megalops. As shown by American fossils, and by several more imperfect 

 specimens from the Cambridge Greensand, the elements within the clavicle are 

 completely fused together, and in addition to the scapula (sc.) and coracoid (cor.) 

 there is a well-developed precoracoid bar (pc). 



Fig. 44. Diagram of left pectoral arch and base of pectoral fin of Protosphyrxna. A. Upper view 

 of base of fin ; B. end view of preaxial part of same ; C. eighth baseost, inner view ; D. pectoral 

 arch, outer view, cor., coracoid ; pc, precoracoid; r., preaxial fin-ray; sc., scapula; i, n, vm, 

 baseosts and (in D.) facet tes for the same. 



Most of the fragments of pectoral fins ascribed by Agassiz 3 to Ptychodus 

 doubtless belong to this species. Basal portions are named Ptychodus arcuatus 

 (loc. cit., p. 58, pi. x a, fig. 2) ; more distal portions are named P. spectabilis (loc. cit., 

 p. 57, pi. x a, fig. 1) ; while small specimens and fragments of distal end are 

 referred to P. gibberulus (he. cit., p. 58, pi. xa, fig. 4). The type specimen of the 

 so-called Ptychodus articulatus (loc. cit., p. 58, pi. X a, figs. 5, 6) is probably part 

 of the caudal fin of a fish of the family Chirocentridas. 



1 E. D. Cope, " Vertebrata of the Cretaceous Formations of the West," Rep. U. S. Geol. Surv. 

 Territ., vol. ii (1875), p. 244 a. 



2 O. P. Hay, Bull. Auier. Mus. Nat. Hist., vol. six (1903), p. 11. 

 n L. Agassiz, Poiss. Foss., vol. iii, 1837, p. 56. 



