142 PROF. P. M. DUNCAN ON AN 



of the mouth-papilla, is the same (but in a greater degree) as that 

 of the ordinary mouth-papillae at the sides of the jaw-angles. All 

 are filterers, and prevent large pieces of substance from enter- 

 ing the digestive cavity. So far as the mouth-papillse of many 

 genera are concerned, their shape and position contraindicate 

 the possibility of any individual movement of the jaw-angles 

 towards or from the oral axis. 



It is evident that the specimen under examination has the 

 true teeth horizontally placed, although the position of the jaws, 

 and necessarily of the jaw-plates, is oblique, and also that these 

 triangular teeth are so closely in apposition that no substance of 

 a visible size can pass up into the stomach. 



In the Ophiuroidea, as a whole, there is a gradation between 

 this almost occluded condition and a wide separation of the points 

 of the true teeth after death. The true teeth are attached at 

 right angles to the long axis of the jaw-plate, which is fixed exter- 

 nally to the jaw, and they project into the canal leading to the 

 stomach according to their size. Sometimes pointed bluntly, 

 they are at others broad, and are either convex or concave at their 

 free internal surface. They are capable of passive up-and-down 

 flap-like movement to a slight degree ; but the statement that 

 they are fixed on to the jaw-plate by muscles I have not been able 

 to verify. Certainly no muscular fibres like those in the arms 

 are to be found ; and the substance which really connects teeth 

 and jaw-plates is connective tissue. The microscopic character 

 of the so-called teeth in the majority of instances indicates that 

 the granules of calcareous matter are arranged more or less in 

 a cellular fashion. There is nothing like a worn surface to be 

 seen on any of the teeth, which in some genera are finely orna- 

 mented with spinules ; and, indeed, usually they have an axial space 

 between their free internal edges. 



There may be a dermal growth below the end of the jaw-angle ; 

 and this may be of all shapes, from the nodular to that of a true 

 tooth. It is a mouth-papilla according to systematic writers, and 

 probably correctly so. Often sufficiently difficult of distinction 

 from the lowest true tooth, this papilla has the same physiological 

 significance. 



In several genera, Ophiothrix being the best example, a num- 

 ber of dermal spinules of different lengths are arranged on the 

 inner side of the lower end of the jaw-plate, in the position of the 

 lowest true teeth ; they are the dental tooth-papillae. Usually 



