VII. 196 F. A. Bather — Studies in Edrioasteroidea. 



formerly presented by the now vanished test as seen from the outside. 

 In this region we see the flooring-plates rising slightly above the 

 interradial areas, then presenting a narrow flat surface, then dipping 

 down in a slightly convex curve to the middle line of the groove. 

 The groove is less deep than in the internal cast, which means that 

 the flooring-plates were thicker in the middle of the groove than 

 at its margins. Between the flooring-plates, where they bend 

 downwards, are elongate slits, wider towards the margins of the 

 groove ; these correspond to the prominences of the internal cast. 



The right anterior and left posterior grooves are still filled in 

 places by portions of matrix ; and the hollows between this matrix 

 and the internal cast enable one to reconstruct a section of the 

 groove based on actual measurement. Fig. 3 is taken from the left 



Fig. 3. — View of the left posterior groove, as preserved in the fossil, x 4 diam. 

 ext. matrix filling the exterior of the groove, int. matrix filling the interior 

 of the test. gr. groove on the surface of the latter, p. matrix filling pores 

 between the flooring- plates, p'. scars where the same has been broken away. 



posterior groove, at 5-5 mm. from its distal end. Here the plates 

 are about 1 mm. thick at the edge of the groove, but thin considerably 

 towards the middle line. 



A good idea of the appearance of the groove as seen from the 

 inside of the test is obtained from a wax squeeze of the internal 

 cast (PI. X, Fig. 5). 



Forbes describes the " margin of the " grooves as composed of 

 " areal or interambulacral plates bearing 2-3 short elevated transverse 

 ridges, each of which points to the origin of an ambulacral plate, 

 short and oblong ; a double series of these ambulacral plates form 

 the canal." It is hard to believe that Forbes did not understand 

 he was dealing with an internal cast ; yet this seems to have been 

 the case, for he proceeds to describe the external impressions of the 

 grooves as triangular arms " composed of two rows of dove-tailing 

 joints, with ridges at the articulations to lock into the furrows 

 bordering the arm-canal." From this truly remarkable misconception 

 arose an extraordinary theory as to the nature of the radial grooves 

 and as to the homologies of the ambulacral areas in Echinoids. 

 After half a century one need not linger over mere errors of fact 

 in the first sentence quoted. The present description and figures 

 will enable readers to make the corrections themselves. 



Piecing all this evidence together as in Fig. 4, we see that each 

 radial groove was composed of a double row of flooring-plates, more 

 or less alternating, and meeting in a median zigzag line. In the 

 proximal half of the groove the long axis of each of these plates, 

 nearly at right angles to the middle line of the groove, measured 

 3-3 "5 mm., and the short axis, parallel to the middle line, *9 mm. 

 The total number of plates along one side of a groove was about 60 



