2l6 AYERS AND JACKSON. [Vol. XVII. 



fin between the outer ends of the ordinary fin-rays, but not 

 connected with them. The dorsal fin is tallest a short distance 

 from its posterior end. Posteriorly it is directly continuous 

 around the posterior end of the body with the ventral fin. The 

 median dorsal bar is continuous with the median ventral bar. 



The ventral fin (Fig. 14, VF) begins in the median line just 

 behind the cloaca, and extends from the seventy-fifth to the 

 ninety-fifth myotome (posterior end of the body). Its fin-rays 

 are from thirty to thirty-five in number. They are all well 

 developed and branched at the outer ends, especially in the pos- 

 terior region, where the branches are in some cases longer than 

 the main stalk. The rays of the ventral fin form two divi- 

 sions. Those of the anterior division (eleven to fifteen) are 

 larger, and approximately segmental in arrangement. Their 

 bases end freely above, and are not directly connected with the 

 notochord or the median ventral bar. From their bases they 

 extend downward and slightly backward in the median line. 

 The first ray is the largest and longest, extending back over 

 the median dorsal roof of the cloaca. 



Beginning about the eighty-fifth myotome, the fin-rays of 

 the posterior division are smaller and more closely crowded 

 together, being about twenty, corresponding to the last ten myo- 

 tomes. Their upper or proximal ends are fused with a large 

 vertical plate, the median ventral bar (Fig. 14, MV). This car- 

 tilage arises anteriorly about the cloacal region as a pair of 

 slender bars running along the infero-lateral angles of the noto- 

 chord in the skeletogenous layer. Anteriorly each of these 

 lateral bars breaks up in some cases into a short chain of small 

 cartilage patches, segmentally arranged, corresponding to the 

 neighboring myotomes. Posteriorly the lateral bars become 

 wider, and finally fuse across the median line below the noto- 

 chord, to form the median ventral bar. This portion of the bar 

 lies above the anterior division of the ventral fin-rays, but is 

 not fused with them. About the region of the eighty-fifth 

 myotome the median ventral bar suddenly enlarges into a ver- 

 tical plate, with which the remaining fin-rays are fused (see 

 Fig. 14), The anterior inferior angle of this plate extends 

 forward as a process (Fig. 14, c) which forms a T'-shaped plate, 



