172 William Sörensen 
described the scouring faces as being »naked« and the most highly 
developed ones as being »naked, smooth, as it were, polished faces«. 
Dr. Tmıto, on the contrary, does not distinguish between articular 
and scouring faces. To his eyes the latter are articular faces as 
well as the former. 
The dorsal fin in the Siluroide. (My book, pag. 6—20.) 
According to my view the rudimentary ray (/) articulates by 
means of a gliding joint (arthrodia plana), not shown in the figures; 
it consists of two pair of artieular faces, of which one is placed on 
the hindside of the muscular erest of the second interspinous bone 
(802), and the other, generally somewhat larger, on the foreside of 
the branches of the rudimentary ray (/) at their base. Behind the 
Fig. 1 (Copy of Fig. 1 of my book). Doras maculatus Cuy. et Val., of a specimen measuring 50 cm 
of length; natural size. The hinder part of the helmet (C) and the muscular crests of the inter- 
spinous bones (Sb2, Sb3) are persected. Laid open to the view are 0, the left side of the roof-like 
keel (the »Gelenkkörper« of Tu1L0) of the second interspinous bone; ar, the articular cavity for the 
median part of the articular face of the spine. 
Fig. 2 (Copy of Fig. 3 of my book). Synodontis schal Bl. The two foremost rays with the skeletal 
parts belonging to them, some of their ligaments (A & 4’), and their muscles (al, a2, p2, ml). 
rudimentary ray is placed a roof-like keel! (0 Fig. 1), the »Doppel- 
kegel« or »Gelenkkörper« of TuıLo, with »naked, smooth, as it 
were, polished faces«, i.e. scouring faces. The excavated hindside 
of the rudimentary ray is furnished with a scouring face, which 
corresponds exactly with those of the roof-like keel. 
1 This keel is rounded in Platystoma; but in Pseudaroides, Euanemus, 
Synodontis, and Doras its scouring faces touch each other at an angle, most 
acute in Doras. 
