CACOPS ASPIDEPHORUS 265 



pygal, hypocentra decrease rapidly in length. With the fifth is a short, 

 rudimentary rib, a mere pointed tubercle, and several similar ribs were 

 found loose in the matrix. 



Beginning with the seventh, or possibly the eighth, caudal there is a 

 continuous series of fourteen or fifteen with chevrons, some of them, as 

 shown in the drawings, with the neural arches attached. It required a 

 critical examination with a lens to distinguish the very small pleuro- 

 centra of the posterior ones ; that I did distinguish them I am quite cer- 

 tain. The hypocentra are angular; the lower posterior part extended 

 into stout chevrons, which were perforated near their base for the hsemal 

 canal. The distal part of the tail was thin and high, possibly used as a 

 rudder-like organ in life. 



RIB8 

 (Plate 12, figure 8) 



Many of the ribs have been removed from their encrusting matrix with 

 but little or no injury; some of the posterior ones, because of their ex- 

 treme delicacy, could not be worked out completely. The first eight pairs 

 have broadly expanded proximal extremities, with a distinct separation, 

 save of the first pair, of the rounded and thickened capitulum from the 

 more elongated and thinner tubercle. The upper border is nearly straight 

 or gently convex on the proximal portion, convex beyond; for the three 

 distal fourths or more the shaft is slender, gently flattened, oval in cross- 

 section, and is curved downward. Beginning with the ninth rib, the 

 proximal expansion is much less, and there is no distinction into capitular 

 and tubercular articulations ; the shaft is more slender. The ribs increase 

 gradually in length to the ninth and probably to the tenth. Beyond this 

 they decrease more rapidly in length. In the last two pairs, at least, 

 they are reduced to tuberculiform rudiments, ending pointedly, and are 

 apparently coosified with the diapophysis. 



From the tenth to the eighteenth only the proximal parts were worked 

 out — that is, their precise lengths could not be determined. 



SIGNIFICANCE OF HYPOCENTRA AND PLEUROCENTRA 



Two views have been and yet are held by morphologists as to the mor- 

 phological significance of the pleurocentra and hypocentra of the rha- 

 chitomous amphibians. The first, that of Cope and Baur,"^ is that in the 

 evolution of the higher vertebrates the pleurocentra progressively devel* 

 oped to form the centrum, while the hypocentrum as progressively 



^ Cope : Transactions of the American Philosophical Society, vol. xvl, p. 243. Baur ; 

 Blologls. Centralbl., vol. vl, 1886, p. 12 ; American Naturalist, 1897, p. 975. 



