202 AYERS AND JACKSON. [Vol. XVII. 



upward, and inward. A little below the level of the floor of 

 the nasal tube, and almost directly below the anterior ends of 

 the cornual cartilages, they give off, on each side, a process 

 which continues outward and forward as the third tentacular 

 cartilage {t^. Like the second pair, these cartilages are coni- 

 cal in shape, tapering gradually to a point. A little further 

 upward and inward, the lateral labial cartilage gives off a short 

 process i^np) which extends inward toward the median line, and 

 abuts against the side of the nasal tube, just above the longi- 

 tudinal connecting bar. This process is attached to the nasal 

 tube by a membranous ligament, but is not joined directly to any 

 cartilages of the nasal tube. The lateral labial cartilage now 

 curves outward, and then turns sharply upward, forward, and 

 inward to terminate in the long, slender, conical cartilage of 

 the first tentacle (/,). It is at the base of this tentacle that the 

 ligament connecting with the anterior end of the cornual carti- 

 lage is attached. The fourth tentacular cartilages (PI. XXII, 

 Fig. 6 ; PI. XXIII, Fig. 7, t^ are remarkable for their shape 

 and position. They are irregular, somewhat elliptical, thin 

 plates of cartilage, nearly vertical in position, and curved later- 

 ally so as to be slightly convex anteriorly. From the upper 

 end a short process extends upward, forward, and outward 

 toward the base of the third tentacle, with which it is connected 

 by a ligament. The cartilaginous plates of the fourth tenta- 

 cles are not fused with any other cartilages, but are supported 

 by the surrounding muscles and connective tissue. It is ques- 

 tionable whether they are true tentacles in the sense that the 

 other three pairs are. In life they appear as slight folds in 

 the skin on the infero-lateral margin of the oral opening. The 

 floor of the skull is formed by a large, strong, trough-shaped 

 cartilaginous apparatus, the basal plate (PI. XXII, Figs. 5, 6; 

 PI. XXIII, Figs. 7-9, B, B', B"). The basal plate may be divided 

 into three segments, an anterior {B), a middle {B'), and a pos- 

 terior {B"). The anterior segment (PI. XXIII, Figs. 8, 9, B) 

 is made up of two pairs of flattened longitudinal bars, an inner 

 and an outer pair. The inner pair {Ai) is the smaller. In the 

 anterior two-thirds they are fused together in the median line, 

 no trace of the fusion showing on the ventral side, and only a 



