382 " ^- POLLARD, 



the skin round the mouth e. g. those of Pîecostomus may not be con- 

 siilered proper tentacles, though they may in a remote way have been 

 connected with tentacles. Such skin processes may be compared with 

 the fringe round a lamprey's mouth. 



Tentacles occur in Cyprinoiils, and especially in Cohitidae, where 

 they attain an equal development with the Siluroids. They appear 

 also in Gadidae and other fish. Motella tricirrafa and Midlus barbatus 

 are familiar examples. Again they are found in Sturgeons, as the 

 barbels under the snout, and in the larva of an Amphibian, Dactyl- 

 ethra {Xenopus). 



Below the fish they appear in Myxinoids, and as 1 maintain, in 

 the well-known form of the oral cirri of Amphioxus. 



A typical tentacle should consist of the following parts : 1) a 

 skeletal axis connected with a root piece, the axis being accompanied 

 by 2) sensory nerves, which supply tactile organs in the skin, and 

 worked by 3) muscles belonging to a special system and not homo- 

 logous with the mctameric body muscles, the 4) motor nerve supply 

 being from nerves which have been shown to arise from the lateral 

 cornu of the central nervous system, and to proceed out with the 

 sensory nerves. 



Descriptive Part. 



Model of Auchenaspis (Figs. 1, 2, 8). 



The head of a specimen, 5 cm in length, was cut in sections 

 30 /< thick. Every second section was drawn with a camera with a 

 magnification of 2S (Zeiss Oc. 2, Obj. aa, height above table of drawing 

 19 cm). Thickness of wax plates 1,35 mm. 



Model 23 cm long (or a little over 9 inches) by 20 cm broad. 



The head as far as reconstructed was about the size of a hazel 

 nut. The specimen was no doubt a young one. The replacement of 

 cartilage by bone has not occurred to any great extent. The head was 

 not modelled further posteriorly than the anterior semicircular canal. 



The anterior semicircular canal is enclosed in cartilage, which 

 does not however extend up as a tegmen cranii, nor is it prolonged 

 downwards to form the floor of the cranial cavity, and there is no 

 appearance of resorption of cartilage, to indicate that at an earlier 

 ontogenetic stage it extended further. A pterotic ridge is present, 

 better developed in its posterior than in its anterior part, and it 

 bears the moderately long articulation for the hyomandibular. The 



