72 BKITISH PALEOZOIC ASTEROZOA. 



seen in this aspect are tlie large odontophors. They are rhomboid in form, 

 and approach more nearly to the primitive rounded shape shown by the American 

 Ordovician species of " Hudsonaster " than do the corresponding plates of any 

 other English Asteroidea in this stage. In some specimens these odontophors 

 are completely enclosed by the neighbouring infero-marginalia ; in others their 

 distal margin is free. It is not clear whether this variability is due to relative 

 movement of the ossicles or to true variations in position. 



The first infero-marginal is much smaller than the odontophor, and from this 

 point the decrease in size of members of the series follows the same course as that 

 of the supero-marginalia. Probably more infero-marginalia are present than those 

 shown in the plan, for the infero-marginalia at the extremity of the arm tend 

 to pass over to the apical surface, and, as mentioned above, this portion of the 

 arm is slender and difl&cult of preservation. 



The mouth-angle plates are small, triangular, and not much larger than the 

 neighbouring adambulacralia. These latter are small, meet across the middle line 

 of the narrow groove, and they are much more numerous than the infero-marginalia. 



Side View. — D. 55 and other specimens show a side view of the arms. 

 Occasionally the proximal supero-marginalia fit exactly over the infero-marginalia, 

 while the distal ossicles of the two series alternate. At other times all the ossicles 

 of the two series alternate. This is in conformity with the power of movement 

 of the ossicles already dealt with. The supero-marginalia are much higher than 

 the infero-marginalia. 



Inferior View. — In making the casts I frequently found the mould filled with 

 a small ball of sandstone, which came away with the cast, but could be detached 

 from it. The cast then showed the inner view of the fossil just as if the apical 

 covering had fallen away and allowed Qiie to look into the body-cavity. Un- 

 fortunately, the small size of the plates does not allow exact description of the 

 various parts. Nevertheless, it is clear that the ambulacralia were exactly opposite, 

 except perhaps those nearest the mouth, where they appear to alternate slightly. 

 The first pair of ambulacralia have the usual slope towards the entrance of the mouth. 



An internal cast of D. 60c allows one to look down on the odontophor which 

 distally is swollen but proximally somewhat flattened and grooved (almost 

 Y-shaped). The arms of the Y distinctly support the ossicles of the mouth-region 

 just as does the odontophor of Recent Asteroidea (compare PI. I, fig. 1). 



Ornament. — All the extra-ambulacral plates are covered with very small 

 granule-like eminences. I have not succeeded in determining the ornament of 

 the adambulacralia. 



Measurements. — Largest specimen, D. 128. R : r : : G'5 mm. (about) : 2*5 mm. 



One of the smaller specimens, D. 169. R. : r : : 4 mm. (about) : 1'8 mm. 



Horizon and Localities. — Upper Ordovician (Ashgillian) of Thraive Olen, 

 Girvan, Ayrshire. 



