236 Scientific Proceedings, Royal t)ublin Society. 



bases, the outermost of each row being the smallest. Twelve to sixteen rows 

 of median teeth and eight or nine rows of lateral teeth compose a radula : 

 the front median teeth are more than twice the size of those placed furthest 

 back, but the lateral teeth exhibit little diminution posteriorly. Hook-sacs 

 very small in proportion to size of teeth and placed at a great distance from 

 radula : sixteen to twenty short, equal-sized hooks of the Pneumodermopsis 

 type are present in each sac. Between the hook-sacs, and much involved in 

 the strong ring-musculature of the gullet, is a row of what appear to be 

 small spines, which from their position would seem to be the jaw. The 

 median tooth of this species is so aberrant in its almost bar-like straightness, 

 and the large number of its denticles, that future research may prove it to be 

 a member of a new family. It is provisionally placed with the Clionopsidae, 

 as the median tooth seems to be nearer that of Clionopsis microcephalus, 

 Tesch, than that of any other species of the Gymnosomata hitherto described. 

 In Tesch's species this tooth is more semi-lunar in shape, and has but five 

 denticles ; the median notch or projection characterizing the median tooth 

 in most of the Clionidae, and Notobranchaeidae, is, however, as in the 

 present species, entirely absent. The median tooth of Thliptodon rotundatus 

 described here (p. 239) is perhaps more like that of the present species, but 

 the form of the lateral teeth and hook-sacs in the family Thliptodontidae is 

 very different. It is interesting, nevertheless, to remember that Thliptodon 

 diaphanus, Meisenheimer, shows the same wide separation between the 

 anterior and posterior lobes of the foot. A posterior lobe to the foot has not 

 hitherto been observed in the Clionopsidae, but Bonnevie (1913, p. 63) 

 attributes only a specific value to the modifications of the foot. Another 

 character favouring the admission of this species to the Clionopsidae lies in 

 the fact that Clionopsis Krohni, Troschel, has the most developed tentacles of 

 any Gymnosomatous Pteropod (Pelseneer, 1887, p. 34). 



Family CLIONIDAE. 

 Cephalobrachia macrochaeta, Bonnevie. 



S.B. 481 — Midwater otter trawl at ca. 600-900 fathoms. One, 8 mm. 



The enormous hook-sacs and radula are so well evaginated in this 

 specimen that they can be seen clearly without dissection. Badula : 3-1-3, 

 agreeing with type in shape, and composed of about ten rows. The curious 

 glandular lip glitters as if powdered with gold dust. The foot and fins are 

 deeply sunk in a skin pocket recalling that of the genus Thliptodon. The 

 viscera completely fill the body. 



Distribution.— 4$,° 24' N., 36° 53' W., 500 metres; 45° 26' N., 25° 45' W. ; 

 750 metres ; 48° 29' N., 13° 55' W., 750 metres (Bonnevie, 1913). 



